United Nations police and security forces clashed on Monday with Serb demonstrators in Kosovo in the worse violence since Kosovo's unilateral declaration of independence  on Feb. 17.

Violence erupted after around 100 UN special police, supported by soldiers from the NATO peacekeeping force KFOR, stormed a UN courthouse in the city of Mitrovica in northern Kosovo at dawn Monday. The courthouse had been occupied since Friday by Kosovo Serb demonstrators protesting Kosovo's independence.

After the UN police seized the courthouse, thousands of Serbs surrounded the compound and clashed with riot police and NATO troops. The UN forces used tear gas and stun grenades to try to ward off the rioters. UN police said 53 Serb demonstrators had been arrested.

According to witnesses, Serb rioters stopped three UN vehicles as they took detained protestors away and managed to release around 20 Serb detainees.

Protestors set at least one UN vehicle and one NATO truck on fire during the riots. Gun fire and explosions were heard in the city and Serbs erected barricades made of cars and trash canisters. The news agency Reuters quoted a French NATO spokesman as saying peacekeepers had come under automatic weapons fire, without providing more concrete details.

Polish national police reported that 22 Polish police officers who had been guarding the UN courthouse were injured after being pelted with grenades and what a police spokesman described as "probably homemade explosive devices." A French military spokesman told the Associated Press that nine members of the international force had been injured, including three French soldiers. Hospital and Kosovo government sources said at least 20 Serb demonstrators had been injured.

The European Commission urged restraint after the clashes. "We are very concerned about the tense situation in north Mitrovica and we regret attacks against the UN police and KFOR," a Commission spokesman told reporters Monday. "We call for restraint. Violence is unacceptable." KFOR is the operating name of the NATO troops deployed in Kosovo.

Mitrovica, which is divided into an ethnic Albanian part and an ethnic Serbian enclave, has been a center of ethnic tension since Kosovo's secession from Serbia, and Serbs wanting to take control of UN-run institutions in Kosovo have protested daily outside the courthouse in recent weeks. Serb protestors have prevented ethnic Albanian judges from going to work at the court, which has been controlled by the UN since the end of the war in Kosovo in 1999. Kosovo Serbs also recently tried to take control of a railway line in northern Kosovo, where most of Kosovo's ethnic Serbs live.