There has been "some loss of life" and dozens of people have been injured after a tram overturned in south London, police have said.
British Transport Police (BTP) said it was "too early to confirm numbers" and two people remain trapped.
It has arrested the tram driver but would not confirm what for. The cause of the crash is unclear.
A number of people were freed following the derailment in Croydon just after 06:00 GMT.
Investigators from the Rail Accident Investigation Branch are at the scene.
Croydon tram overturns: 'Some loss of life' and two trapped
BBC News
8 November 2016
There has been "some loss of life" and dozens of people have been injured after a tram overturned in south London, police have said.
British Transport Police (BTP) said it was "too early to confirm numbers" and two people remain trapped.
It has arrested the tram driver but would not confirm what for. The cause of the crash is unclear.
A number of people were freed following the derailment in Croydon just after 06:00 GMT.
Investigators from the Rail Accident Investigation Branch are at the scene.
London Ambulance Service said 51 people had been taken to St George's Hospital in Tooting, south London, and Croydon University Hospital.
St George's said it was treating 20 people, including four who were seriously injured.
Firefighters are working to free two people who remain trapped in the tram
The overturned tram remains on its side next to an underpass and appears to have derailed where the track branches.
Robin Smith, assistant chief constable of BTP, said: "A number of people have been taken to hospital with injuries and sadly it looks as though there has been some loss of life.
"It is too early for us to confirm numbers but we are working hard to assess the ongoing incident and we are continuing to focus on recovery efforts."
People living nearby said they heard a loud crash and saw the injured being taken away by stretcher
Scores of emergency services are attending the incident
Local resident Hannah Collier said: "I was in bed watching the election when I heard a big crash, which I thought was the wind.
"Then I heard people shouting and then the emergency services arriving.
"Later I saw at least one person being brought out on a stretcher."
The tram overturned at the Sandilands stop
Prime Minister Theresa May offered her thoughts and prayers and said the government was "closely in touch with the emergency services and other authorities to ensure that all is being done that is necessary to deal with this terrible incident."
Mayor of London Sadiq Khan said: "My thoughts and prayers are with the friends and families of those who have lost their lives today, and everyone involved in this incident.
"I am in close contact with the emergency services and Transport for London, who are working extremely hard under very difficult circumstances to get the situation under control and treat those who have been injured.
"The police have set up a number for friends and family to call for information: 0800 0560154."
A book of condolence has opened at Croydon Town Hall.
At the scene: Tom Edwards, BBC London's transport correspondent
We have no idea how it happened.
What we have been told is that there is a steep, quite a bad bend so the tram may have derailed.
There was also torrential rain earlier perhaps that had something to do with it - but this is all complete speculation.
It's unbelievable that this has happened - it's unprecedented really since the tram opened in 2000.
We haven't had an incident of this scale for some time.
The whole area has come to a standstill.
The mood is extremely sombre.
Croydon Tramlink
London's only tram network operates from Wimbledon to Beckenham Junction, Elmers End and New Addington, via Croydon
It began operation in May 2000 as Croydon Tramlink, becoming the first tram system in London since 1952
More than 27 million passengers used the service in 2015/16
The 17 miles (28km) network consists of 39 stops over
Croydon tram overturns: 'Some loss of life' and two trapped - BBC News
British Transport Police (BTP) said it was "too early to confirm numbers" and two people remain trapped.
It has arrested the tram driver but would not confirm what for. The cause of the crash is unclear.
A number of people were freed following the derailment in Croydon just after 06:00 GMT.
Investigators from the Rail Accident Investigation Branch are at the scene.
Croydon tram overturns: 'Some loss of life' and two trapped
BBC News
8 November 2016
There has been "some loss of life" and dozens of people have been injured after a tram overturned in south London, police have said.
British Transport Police (BTP) said it was "too early to confirm numbers" and two people remain trapped.
It has arrested the tram driver but would not confirm what for. The cause of the crash is unclear.
A number of people were freed following the derailment in Croydon just after 06:00 GMT.
Investigators from the Rail Accident Investigation Branch are at the scene.
London Ambulance Service said 51 people had been taken to St George's Hospital in Tooting, south London, and Croydon University Hospital.
St George's said it was treating 20 people, including four who were seriously injured.
Firefighters are working to free two people who remain trapped in the tram
The overturned tram remains on its side next to an underpass and appears to have derailed where the track branches.
Robin Smith, assistant chief constable of BTP, said: "A number of people have been taken to hospital with injuries and sadly it looks as though there has been some loss of life.
"It is too early for us to confirm numbers but we are working hard to assess the ongoing incident and we are continuing to focus on recovery efforts."
People living nearby said they heard a loud crash and saw the injured being taken away by stretcher
Scores of emergency services are attending the incident
Local resident Hannah Collier said: "I was in bed watching the election when I heard a big crash, which I thought was the wind.
"Then I heard people shouting and then the emergency services arriving.
"Later I saw at least one person being brought out on a stretcher."
The tram overturned at the Sandilands stop
Prime Minister Theresa May offered her thoughts and prayers and said the government was "closely in touch with the emergency services and other authorities to ensure that all is being done that is necessary to deal with this terrible incident."
Mayor of London Sadiq Khan said: "My thoughts and prayers are with the friends and families of those who have lost their lives today, and everyone involved in this incident.
"I am in close contact with the emergency services and Transport for London, who are working extremely hard under very difficult circumstances to get the situation under control and treat those who have been injured.
"The police have set up a number for friends and family to call for information: 0800 0560154."
A book of condolence has opened at Croydon Town Hall.
At the scene: Tom Edwards, BBC London's transport correspondent
We have no idea how it happened.
What we have been told is that there is a steep, quite a bad bend so the tram may have derailed.
There was also torrential rain earlier perhaps that had something to do with it - but this is all complete speculation.
It's unbelievable that this has happened - it's unprecedented really since the tram opened in 2000.
We haven't had an incident of this scale for some time.
The whole area has come to a standstill.
The mood is extremely sombre.
Croydon Tramlink
London's only tram network operates from Wimbledon to Beckenham Junction, Elmers End and New Addington, via Croydon
It began operation in May 2000 as Croydon Tramlink, becoming the first tram system in London since 1952
More than 27 million passengers used the service in 2015/16
The 17 miles (28km) network consists of 39 stops over
Croydon tram overturns: 'Some loss of life' and two trapped - BBC News
