U.N. forces pulling Serb demonstrators from a U.N. courthouse were attacked Monday by hundreds of furious protesters who massed outside, setting off an hours-long battle with rocks, grenades and live ammunition.

U.N. police and NATO troops responded with tear gas, stun grenades and gunfire. At least 45 U.N. and NATO forces and 70 protesters were injured in the worst violence in Kosovo since its majority ethnic Albanians declared independence from Serbia a month ago.

NATO and the U.N. issued a joint statement in Kosovo's capital, Pristina, condemning "lethal violence, including direct fire by a mob" in the Serb-controlled part of the town of Kosovska Mitrovica.

About 100 U.N. policemen and NATO troops arrested 53 Serbs who had occupied the courthouse in Mitrovica since Friday, U.N. police said. The group had seized the building to protest Kosovo's declaration of independence, which the territory's Serb minority fiercely opposed.

Since the Feb. 17 declaration, Serb protesters have been trying to take control of local institutions that have been run by the U.N. since the end of the war in Kosovo in 1999. Crowds have gathered daily at the courthouse in Mitrovica to prevent international and ethnic Albanian judges from returning to work there.

Later Monday, hundreds of U.N. police pulled out of the Serb-controlled northern part of the city after apparently coming under small arms fire by Serbs. NATO troops remained in the Serb part of the town, which is divided between rival Kosovo Serbs and Albanians, to try to restore order.

Witnesses said about 20 of the Serbs arrested Monday were later freed by Serb rioters who stopped three U.N. vehicles driving them away.

"After the special U.N. police burst into the court, they bashed some furniture and handcuffed us," said Milenko Stranic, one of the Serbs who escaped arrest. "They refused to speak to us and impounded our mobile phones."

At least one U.N. vehicle and one NATO truck were set on fire.

Among the international forces wounded were 27 Polish officers, 15 Ukrainians and three French, according to officials from those countries. Danish military police arriving to evacuate the wounded came under fire, but none was injured, a spokesman for the Danish force said.

At least 70 Serb demonstrators, including one struck in the eye by a bullet, were also injured, hospital officials said.

"Most of the civilians suffered injuries from shock bombs, tear gas and explosive devices," said Vladimir Adzic, the head of a nearby hospital.

In Mitrovica, Serbia's government minister for Kosovo, Slobodan Samardzic, described the action by U.N. police as "brutal and inadmissible."

In Belgrade, Serbian President Boris Tadic accused the international forces in Kosovo of "using excessive force," and warned of "escalation of clashes in the entire territory" of Kosovo.

Tadic urged the U.N. and NATO to refrain from using force and called on the Serbs not to provoke the international forces. He urged Kosovo Serb leaders to do all they can to restore calm.

The European Union expressed concern about the violence and called for restraint.

In Mitrovica, NATO helicopters hovered above the town, which is divided by the Ibar River between Kosovo Albanians and Serbs. The Serbs turned on an air-raid alarm during the clashes.

Alexander Ivanko, spokesman for the U.N. mission in Kosovo, said the U.N. staff in Kosovska Mitrovica "have been ordered to relocate" to the regional headquarters in the southern part of the town.

He confirmed that the NATO force, known as KFOR, would take control of the Serb sector in northern Mitrovica, but could not say if KFOR was bringing in reinforcements.

Kosovo's deputy prime minister defended the peacekeepers' action, demanding that they do not end their operation.

"We have requested from the first day that U.N. and KFOR establish the rule of law in north Mitrovica and to protect institutions there. It was a just action and the right one," Hajredin Kuqi said. "There can be no compromise when it comes to the rule of law."