UN and NATO security forces clashed violently with Serbian protesters on Monday after storming the UN courthouse in northern Kosovo to arrest demonstrators who had occupied the building since last week.

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A UN vehicle burns as French NATO peacekeeping troops, background, are seen during clashes in the Serb-dominated part of the ethnically divided town of Kosovska Mitrovica, Kosovo, on Monday. ((Zveki/Associated Press))

Hundreds of Serbs surrounded the courthouse as the police tried to leave with the demonstrators who had been arrested. Some threw rocks, hand grenades and Molotov cocktails and fired guns at the international forces.

They responded with tear gas, stun grenades and gunshots. At least 42 UN and NATO forces and 70 protesters were wounded, reports said.

The clashes ended by noon, but hundreds of UN police decided to pull out of northern Mitrovica after coming under small-arms fire, leaving NATO troops to try to restore order.

Earlier, about 100 UN riot police and NATO troops arrested dozens of the demonstrators who took over the court last Friday in Kosovska Mitrovica to protest Kosovo's declaration of independence last month.

"The operation was undertaken by UN special police units backed by NATO peacekeepers," Veton Elshani, a spokesman for the UN police, told the Associated Press. "The 53 persons were arrested for the illegal occupation of premises controlled by the UN mission here."

At least one UN vehicle and one NATO truck were set on fire during the riots.

Witnesses said 20 of the 53 Serb demonstrators who were arrested in the morning were later released when the rioters stopped UN vehicles driving them away.

With files from the Associated Press