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UN police in Kosovo have been forced to withdraw from the Serb stronghold in northern Mitrovica after riots in which several policemen were injured.
About 70 Serb civilians were injured as well as at least 36 members of the UN police or the Nato-led military K-For.
The clashes began after the UN tried to wrest control of a courthouse seized by the Serbs last week.
It is the worst unrest since Kosovo's independence declaration last month which Serbia says is illegal.
It also coincides with the fourth anniversary of rioting in Mitrovica by Kosovo Albanians which drove some 4,000 people, mainly Serbs, from their homes, and left at least 19 people dead, according to UN figures.
K-For has taken over security in the town where Serbs are concentrated on the northern side of the river.
Nato says it will deal "firmly" with any further acts of violence.
At least 70 Serbs were treated for injuries, including one person with a gunshot wound to the head, according to Kosovo Serb hospital sources quoted by Serbian media and the Associated Press news agency.
Most of the UN or K-For personnel injured were Polish or Ukrainian.
The violence began after about 100 UN police arrested 53 Serbs occupying a UN court in the north of the city.
Scores of protesters blocked the police vehicles as they tried to leave and rocks and petrol bombs were thrown, according to Kosovo police.
Almost half of those arrested were set free during the violence, and UN and Nato vehicles were set alight.
Three UN policemen and two K-For soldiers were injured in an explosion, thought to have been caused by a hand grenade, and there were reports of gunfire in the town.
As the situation escalated, UN police were ordered to withdraw, leaving Nato K-For troops to control the situation.
Speaking in Brussels, Nato spokesman James Appathurai said troops would "respond firmly to ensure a safe and secure environment".
The rioters who have used Molotov cocktails, grenades and possible automatic weapons fire have gravely violated the law," he added.
The European Commission voiced its full support for the efforts of the UN administration and K-For to maintain order, saying violence was unacceptable.
Serbian President Boris Tadic accused the international forces in Kosovo of using "excessive force" and warned of "an escalation of unrest on all the territory of the province".
Many of the protesters who seized the court last week are said to be former staff who lost their jobs in 1999 at the end of the war in Kosovo, when it came under UN administration.
Serbs had staged rallies outside the building for several weeks, preventing ethnic Albanian court employees from crossing the bridge over the Ibar River that divides Mitrovica into a Serb-run north and an Albanian south.
Tension in the region has risen sharply since Kosovo declared independence.
Last week, Serbs tried to take control of a railway line in northern Kosovo.
In February, some 150 Kosovo Serb police officers were suspended for refusing to take orders from the Kosovo Albanian authorities in Kosovo's capital, Pristina.
Most EU states and the US have recognised Pristina's unilateral declaration of independence.
Serbia - backed by its ally Russia - says the move is illegal.