Thousands of people, including members of the Royal Family, are at a ceremony in France to mark the centenary of the start of the Battle of the Somme.
The Prince of Wales and Duchess of Cornwall, the Duke and Duchess of Cambridge and Prince Harry are at the Thiepval Memorial for the event.
It follows a nationwide two-minute silence at 07:28 BST that marked when the battle began on 1 July 1916.
More than a million men were killed and wounded on all sides in the WWI battle.
The Battle of the Somme, one of World War One's bloodiest, was fought in northern France and lasted five months, with the British suffering almost 60,000 casualties on the first day alone.
The British and French armies fought the Germans in a brutal battle of attrition on a 15-mile front.
At a vigil in France on Thursday, the Duke of Cambridge paid tribute to the fallen soldiers, saying "we lost the flower of a generation".
Battle of the Somme: Royals at Somme centenary commemoration
BBC News
1 July 2016
Thousands of people, including members of the Royal Family, are at a ceremony in France to mark the centenary of the start of the Battle of the Somme.
The Prince of Wales and Duchess of Cornwall, the Duke and Duchess of Cambridge and Prince Harry are at the Thiepval Memorial for the event.
It follows a nationwide two-minute silence at 07:28 BST that marked when the battle began on 1 July 1916.
More than a million men were killed and wounded on all sides in the WWI battle.
The Battle of the Somme, one of World War One's bloodiest, was fought in northern France and lasted five months, with the British suffering almost 60,000 casualties on the first day alone.
The British and French armies fought the Germans in a brutal battle of attrition on a 15-mile front.
At a vigil in France on Thursday, the Duke of Cambridge paid tribute to the fallen soldiers, saying "we lost the flower of a generation".
At an early-morning ceremony at the Lochnagar crater, which was created by an explosion at the start of the battle in La Boiselle, France, a rocket was fired to simulate the artillery fire.
This was followed by whistles to symbolise those that were blown 100 years ago as men scrambled from the trenches.
Members of the Royal Family joined French President Francois Hollande to pay their respects at Thiepval
French President Francois Hollande, UK Prime Minister David Cameron, the Duke and Duchesses of Cornwall and Irish President Michael D Higgins were among the dignitaries attending an event at the Commonwealth War Graves Commission memorial at Thiepval
Soldiers of the Irish Guards stand to attention at Thiepval
A soldier stands among the crosses of the missing at Thiepval
A vigil was maintained at the Thiepval memorial in northern France until Friday morning to mark the moment troops first went into battle
The Duke and Duchess of Cambridge and Prince Harry attended part of a military-led vigil at Thiepval near the Somme battlefield. Prince William said "we lost the flower of a generation"
New Zealand army chief, Maj Gen Peter Kelly, is seen walking among gravestones at Thiepval
Ahead of the two-minute silence in the UK, the King's Troop Royal Horse Artillery fired guns from Parliament Square for 100 seconds to mark the 100 years since the battle began.
Across the country and at the vigil sites at Westminster Abbey, Edinburgh Castle, the Somme Heritage Centre in County Down, the Welsh National War Memorial in Cardiff, as well as in France, the silence was observed.
Later, leaders from Northern Ireland and the Republic of Ireland will attend a service at the Ulster Tower, a memorial to the men of the 36th (Ulster) Division in Thiepval, at 13:30 BST.
At the preserved trenches at Newfoundland Park Memorial in Beaumont-Hamel, France, there will be a ceremony to mark the Canadians' part in the battle at 15:30 BST.
The Prince of Wales and Duchess of Cornwall will also attend these ceremonies.
The Duchess of Cornwall will lay a wreath at the grave of her great-uncle, Captain Harry Cubitt, who was killed on the Somme in September 1916 while serving with the Coldstream Guards.
He was the eldest, and the first, of three brothers to die serving on the Western Front.
The 100th anniversary will be marked by Germany in Fricourt, France, where 40,000 Germans are buried.
And in Manchester, there will be a national service of commemoration at the cathedral from 15:00 BST and a concert at 19:30.
A gun was fired to mark the end of the vigil at Westminster Abbey
Meanwhile, men dressed as World War One soldiers have been spotted in railway stations and on UK streets handing out cards detailing casualties of the Battle of the Somme.
The hashtag #wearehere, which features on the cards, is trending on Twitter as pictures of the tribute are shared.
Men dressed as World War One soldiers have been spotted at railway stations
At the Westminster Abbey service on Thursday, the Queen was joined by the Duke of Edinburgh as she laid flowers at the Tomb of the Unknown Warrior.
The tomb holds an unidentified British soldier killed on a European battlefield, brought back and buried in the abbey to honour the unknown dead of the war.
A ceremony at the Lochnagar Crater in La Boiselle, France, was the first event of the day
A wreath was laid at the Tomb of the Unknown Warrior, where a vigil was held
Military personnel and school pupils stood in reflective silence at the tomb. The battle saw more than one million men killed and wounded on all sides
The Bishop of London, the Right Reverend Dr Richard Chartres, said the legacy should be that people worked towards reconciliation to ensure children never endured what the soldiers of World War One faced.
Society must strive to reach an accord and reject "those who would stir up hatred and division," he said.
Prince Harry gave a reading of Before Action by Lieutenant William Noel Hodgson
The Duke and Duchess of Cambridge and Prince Harry paid their respects in France, attending a vigil at the Thiepval Memorial on Thursday evening, located close to the battlefields of the Somme, near Amiens in the north of the country.
Prince William spoke of European governments "including our own" who failed to "prevent the catastrophe of world war".
"We lost the flower of a generation; and in the years to come it sometimes seemed that with them a sense of vital optimism had disappeared forever from British life," he said.
"It was in many ways the saddest day in the long story of our nation."
A vigil was held at Westminster Abbey
The Queen laid a wreath at the Tomb of the Unknown Warrior
Prince Harry also spoke at the event, reading the poem Before Action, by Lieutenant WN Hodgson of the 9th Battalion the Devonshire Regiment, who wrote it before he was killed in action on 1 July 1916.
Before the vigil, the three royals climbed to the top of the huge, newly renovated monument designed by Sir Edwin Lutyens to view the battlefield.
The memorial bears the names of more than 70,000 British and South African soldiers who have no known grave.
Welsh First Minister Carwyn Jones joined personnel from the Army, Royal Navy and the Royal Air Force for the start of a vigil at Llandaff Cathedral in Cardiff.
"Those who fought bravely for our futures should never be forgotten," he said.
In Scotland, an overnight vigil was held at the National War Memorial.
And in Northern Ireland, a vigil was held at the Somme Museum near Newtownards, County Down. A guard of honour, including serving soldiers, was present throughout the night.
The Battle of the Somme was intended to achieve a decisive victory for the British and French against Germany's forces.
The British army was forced to play a larger than intended role after the German attack on the French at Verdun in February 1916.
World War One finally ended in November 1918.
Bad weather was turning much of the battlefield into a quagmire. The vermin-ridden trenches were havens for dirt and disease and battle was finally brought to a halt on 18 November.
In France
By Robert Hall, BBC News correspondent
Across the rolling Somme landscape, the whistles shrilled again; a century ago they sent tens of thousands out of their trenches, and across No Man's Land.
Today they were sounded on the lip of the Lochnagar crater, 300ft wide and 70 deep, the result of a British attempt to breach German defences.
Sixty thousand pounds of explosive sent debris 4,000ft into the air; no-one knows how many were killed.
At the cross of remembrance, a carpet of wreaths was laid, by representatives from Britain, France and Germany, along with families and local children.
In the base of the crater, beside a giant poppy, a lone bugler sounded the last post as clouds of poppies fluttered down on the breeze.
The Battle of the Somme
Began on 1 July 1916 and was fought along a 15-mile front near the River Somme in northern France
19,240 British soldiers died on the first day - the bloodiest day in the history of the British army
The British captured just three square miles of territory on the first day
At the end of hostilities, five months later, the British had advanced just seven miles and failed to break the German defence
In total, there were more than a million dead and wounded on all sides, including 420,000 British, about 200,000 from France and an estimated 465,000 from Germany
Battle of the Somme: Royals at Somme centenary commemoration - BBC News
The Prince of Wales and Duchess of Cornwall, the Duke and Duchess of Cambridge and Prince Harry are at the Thiepval Memorial for the event.
It follows a nationwide two-minute silence at 07:28 BST that marked when the battle began on 1 July 1916.
More than a million men were killed and wounded on all sides in the WWI battle.
The Battle of the Somme, one of World War One's bloodiest, was fought in northern France and lasted five months, with the British suffering almost 60,000 casualties on the first day alone.
The British and French armies fought the Germans in a brutal battle of attrition on a 15-mile front.
At a vigil in France on Thursday, the Duke of Cambridge paid tribute to the fallen soldiers, saying "we lost the flower of a generation".
Battle of the Somme: Royals at Somme centenary commemoration
BBC News
1 July 2016
Thousands of people, including members of the Royal Family, are at a ceremony in France to mark the centenary of the start of the Battle of the Somme.
The Prince of Wales and Duchess of Cornwall, the Duke and Duchess of Cambridge and Prince Harry are at the Thiepval Memorial for the event.
It follows a nationwide two-minute silence at 07:28 BST that marked when the battle began on 1 July 1916.
More than a million men were killed and wounded on all sides in the WWI battle.
The Battle of the Somme, one of World War One's bloodiest, was fought in northern France and lasted five months, with the British suffering almost 60,000 casualties on the first day alone.
The British and French armies fought the Germans in a brutal battle of attrition on a 15-mile front.
At a vigil in France on Thursday, the Duke of Cambridge paid tribute to the fallen soldiers, saying "we lost the flower of a generation".
At an early-morning ceremony at the Lochnagar crater, which was created by an explosion at the start of the battle in La Boiselle, France, a rocket was fired to simulate the artillery fire.
This was followed by whistles to symbolise those that were blown 100 years ago as men scrambled from the trenches.
Members of the Royal Family joined French President Francois Hollande to pay their respects at Thiepval
French President Francois Hollande, UK Prime Minister David Cameron, the Duke and Duchesses of Cornwall and Irish President Michael D Higgins were among the dignitaries attending an event at the Commonwealth War Graves Commission memorial at Thiepval
Soldiers of the Irish Guards stand to attention at Thiepval
A soldier stands among the crosses of the missing at Thiepval
A vigil was maintained at the Thiepval memorial in northern France until Friday morning to mark the moment troops first went into battle
The Duke and Duchess of Cambridge and Prince Harry attended part of a military-led vigil at Thiepval near the Somme battlefield. Prince William said "we lost the flower of a generation"
New Zealand army chief, Maj Gen Peter Kelly, is seen walking among gravestones at Thiepval
Ahead of the two-minute silence in the UK, the King's Troop Royal Horse Artillery fired guns from Parliament Square for 100 seconds to mark the 100 years since the battle began.
Across the country and at the vigil sites at Westminster Abbey, Edinburgh Castle, the Somme Heritage Centre in County Down, the Welsh National War Memorial in Cardiff, as well as in France, the silence was observed.
Later, leaders from Northern Ireland and the Republic of Ireland will attend a service at the Ulster Tower, a memorial to the men of the 36th (Ulster) Division in Thiepval, at 13:30 BST.
At the preserved trenches at Newfoundland Park Memorial in Beaumont-Hamel, France, there will be a ceremony to mark the Canadians' part in the battle at 15:30 BST.
The Prince of Wales and Duchess of Cornwall will also attend these ceremonies.
The Duchess of Cornwall will lay a wreath at the grave of her great-uncle, Captain Harry Cubitt, who was killed on the Somme in September 1916 while serving with the Coldstream Guards.
He was the eldest, and the first, of three brothers to die serving on the Western Front.
The 100th anniversary will be marked by Germany in Fricourt, France, where 40,000 Germans are buried.
And in Manchester, there will be a national service of commemoration at the cathedral from 15:00 BST and a concert at 19:30.
A gun was fired to mark the end of the vigil at Westminster Abbey
Meanwhile, men dressed as World War One soldiers have been spotted in railway stations and on UK streets handing out cards detailing casualties of the Battle of the Somme.
The hashtag #wearehere, which features on the cards, is trending on Twitter as pictures of the tribute are shared.
Men dressed as World War One soldiers have been spotted at railway stations
At the Westminster Abbey service on Thursday, the Queen was joined by the Duke of Edinburgh as she laid flowers at the Tomb of the Unknown Warrior.
The tomb holds an unidentified British soldier killed on a European battlefield, brought back and buried in the abbey to honour the unknown dead of the war.
A ceremony at the Lochnagar Crater in La Boiselle, France, was the first event of the day
A wreath was laid at the Tomb of the Unknown Warrior, where a vigil was held
Military personnel and school pupils stood in reflective silence at the tomb. The battle saw more than one million men killed and wounded on all sides
The Bishop of London, the Right Reverend Dr Richard Chartres, said the legacy should be that people worked towards reconciliation to ensure children never endured what the soldiers of World War One faced.
Society must strive to reach an accord and reject "those who would stir up hatred and division," he said.
Prince Harry gave a reading of Before Action by Lieutenant William Noel Hodgson
The Duke and Duchess of Cambridge and Prince Harry paid their respects in France, attending a vigil at the Thiepval Memorial on Thursday evening, located close to the battlefields of the Somme, near Amiens in the north of the country.
Prince William spoke of European governments "including our own" who failed to "prevent the catastrophe of world war".
"We lost the flower of a generation; and in the years to come it sometimes seemed that with them a sense of vital optimism had disappeared forever from British life," he said.
"It was in many ways the saddest day in the long story of our nation."
A vigil was held at Westminster Abbey
The Queen laid a wreath at the Tomb of the Unknown Warrior
Prince Harry also spoke at the event, reading the poem Before Action, by Lieutenant WN Hodgson of the 9th Battalion the Devonshire Regiment, who wrote it before he was killed in action on 1 July 1916.
Before the vigil, the three royals climbed to the top of the huge, newly renovated monument designed by Sir Edwin Lutyens to view the battlefield.
The memorial bears the names of more than 70,000 British and South African soldiers who have no known grave.
Welsh First Minister Carwyn Jones joined personnel from the Army, Royal Navy and the Royal Air Force for the start of a vigil at Llandaff Cathedral in Cardiff.
"Those who fought bravely for our futures should never be forgotten," he said.
In Scotland, an overnight vigil was held at the National War Memorial.
And in Northern Ireland, a vigil was held at the Somme Museum near Newtownards, County Down. A guard of honour, including serving soldiers, was present throughout the night.
The Battle of the Somme was intended to achieve a decisive victory for the British and French against Germany's forces.
The British army was forced to play a larger than intended role after the German attack on the French at Verdun in February 1916.
World War One finally ended in November 1918.
Bad weather was turning much of the battlefield into a quagmire. The vermin-ridden trenches were havens for dirt and disease and battle was finally brought to a halt on 18 November.
In France
By Robert Hall, BBC News correspondent
Across the rolling Somme landscape, the whistles shrilled again; a century ago they sent tens of thousands out of their trenches, and across No Man's Land.
Today they were sounded on the lip of the Lochnagar crater, 300ft wide and 70 deep, the result of a British attempt to breach German defences.
Sixty thousand pounds of explosive sent debris 4,000ft into the air; no-one knows how many were killed.
At the cross of remembrance, a carpet of wreaths was laid, by representatives from Britain, France and Germany, along with families and local children.
In the base of the crater, beside a giant poppy, a lone bugler sounded the last post as clouds of poppies fluttered down on the breeze.
The Battle of the Somme
Began on 1 July 1916 and was fought along a 15-mile front near the River Somme in northern France
19,240 British soldiers died on the first day - the bloodiest day in the history of the British army
The British captured just three square miles of territory on the first day
At the end of hostilities, five months later, the British had advanced just seven miles and failed to break the German defence
In total, there were more than a million dead and wounded on all sides, including 420,000 British, about 200,000 from France and an estimated 465,000 from Germany
Battle of the Somme: Royals at Somme centenary commemoration - BBC News
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