The riots posed a direct challenge to Nato, the United Nations and Kosovo's fledgling European Union justice mission, underscoring fears in the West that Kosovo could be heading for ethnic partition one month after breaking away from Serbia.
A UN spokesman said the riot "crosses one of the red lines that had clearly been articulated by the UN to the leaders of Kosovo Serbs in the north and to officials in Belgrade". The United Nations said at least 25 police officers were hurt.
But Serbia blamed the United Nations and Nato for heavy-handed action and increased the level of security on its borders, warning that the volatile situation risked provoking a fresh Albanian "pogrom" against Kosovo's 120,000 minority Serbs.
The Orthodox church said Serbs were "again being killed".
Serbia's main ally Russia blamed the "illegitimate" secession of Kosovo for the rioting and urged the international police contingent to show restraint.
"A turn of events which leads to violence and clashes cannot be allowed," Russia's Foreign Ministry said in a statement. "The international presence should show restraint and act strictly in accordance with its [United Nations] mandate."
The Serb director of Mitrovica hospital, political hardliner Marko Jaksic, said three people of Serb nationality had been severely injured in the rioting.
"One person has been shot in the head most probably by a sniper. The bullet went right through," he said. "The person is in very bad condition [in] a hospital in Kragujevac in central Serbia." Two others were in serious condition, Jaksic said.
The loyalist Serbian National Council of northern Kosovo called on Belgrade to "help Serb people and say that Kosovo is Serbia", its leader Nebojsa Jovic was quoted as saying.
In an apparent hint at the forced partition the West fears, he said "there is a scenario" if people get killed, which could reach "a point from which there's no way back".
The violence began at dawn when several hundred UN special police backed by Nato peacekeepers stormed a UN court that had been seized by Serbs on Friday, and arrested dozens.
Hundreds of Serbs fought back with stones, grenades and firecrackers, forcing the UN police to pull back and leave KFOR to face the rioters. Rioters attacked three UN vehicles, breaking doors and freeing around 10 of those detained in the raid, witnesses said.
Police and troops responded with tear gas. Some UN vans with detainees were still in the courtyard of the compound, with dozens of Serb protesters outside blocking their exit.
Nato said its troops had come under automatic weapons fire.
"We used automatic weapons to respond but fired only warning shots," French spokesman Etienne du Fayet de la Tour told Reuters. "We shot in the air, not into the crowd."
"Eight French KFOR soldiers are injured with grenades, stones and Molotov cocktails," said du Fayet de la Tour.
UN police withdrew "after attacks with explosive devices suspected to be hand grenades, and firearms", a statement said.
Fourteen Ukrainian police serving with the United Nations were injured when "fighters attacked a police station", Ukrainian Interior Minister Yuri Lutsenko told reporters in Simferopol. Poland said 13 of its Kosovo UN officers were hurt.
The raid to retake the court coincided with the March 17 anniversary of Kosovo Albanian riots against Serbs in 2004, in which 19 people were killed and hundreds of homes and churches burned in two days of chaos that caught Nato flat-footed.
It was this flare-up that pushed the West to start talks on Kosovo's final status in 2006. But they got nowhere.
Serbia's President Boris Tadic warned Nato and the United Nations of the risk for Kosovo Serbs on the anniversary of what he called an Albanian "pogrom".
Bishop Amfilohije, acting head of the Serb Orthodox church, said "the suffering of our people is continuing this morning in Mitrovica... strongmen are continuing to kill our people".
The Serbian Interior Ministry "raised the security level in the territory of Serbia to a higher level", the state news agency Tanjug quoted a ministry source as saying.
In Belgrade, police reinforced protection at embassies and government buildings ahead of planned protests on the March 2004 anniversary. (Reuters)
UN and Nato troops clash with Serbs in Kosovo