Updated

Hundreds of Serbs stormed a U.N. courthouse in northern Kosovo on Friday, taking control of the site and hoisting a Serbian flag to replace a U.N. one.

They broke through two gates and pushed aside U.N. riot police guarding the building in the Serb-dominated city of Kosovska Mitrovica, a police spokesman said. Dozens of U.N. police did not intervene.

The top U.N. official in Kosovo said he has ordered police to retake the courthouse, and pledged to defend his mandate as head of the U.N. mission known as UNMIK.

"Those who turned to violence in North Mitrovica have crossed one of UNMIK's red lines. This is completely unacceptable," Joachim Ruecker said in a statement.

"I have instructed UNMIK police to restore law and order in the north and to ensure that the courthouse is again under U.N. control."

He said the attackers would be prosecuted, and called upon Serbian authorities to prevent further such incidents.

The EU also "strongly condemned" the court seizure and called on Serbia to refrain from any actions that may jeopardize security in Kosovo.

"No one benefits from the use of violence and from the attacks on the rule of law institutions in Kosovo," according to a statement by Slovenia, which holds the rotating EU presidency.

Most of the protesters left the yard of the building in the afternoon and the U.N. flag was raised again, U.N. officials said. Serb judicial workers, however, remained locked inside the courthouse, saying they would not leave without a deal with U.N. authorities.

U.N. special police units were on standby to take control of the court and remove protesters.

NATO's Secretary General Jaap de Hoop Scheffer, meanwhile, arrived for his first visit to Kosovo since it declared independence last month.

Scheffer was scheduled to meet Prime Minister Hashim Thaci and President Fatmir Sejdiu at NATO's Kosovo headquarters in the capital, Pristina.

Sejdiu said he has urged NATO and the U.N. authorities to "react urgently and oust the hooligans from the building."

A spokesman for Kosovo's police in Kosovska Mitrovica said the regional U.N. representative was negotiating with Serb leaders to deal with the situation.

The storming of the U.N. courthouse appeared to have been coordinated with the Serbian government in Belgrade, which has rejected Kosovo's declared statehood and said it will assume authority in northern Kosovo.

Belgrade has adopted the so-called "Action Plan" on Kosovo, which took effect after Kosovo declared independence. Although concrete measures proposed in the plan remain secret, some of them were leaked by the local media, and they allegedly include the takeover of the judiciary in the Serb-controlled regions of Kosovo.

Serbia did not immediately comment on the allegations that it may have coordinated the seizure of the courthouse.

The Serbs have held daily protests in front of the court since Kosovo declared independence from Serbia on Feb. 17.

The 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. The crowds have prevented international and ethnic Albanian judges from returning to work at the court.

"We tried to negotiate, but no one wanted to talk to us," said Miodrag Ralic, one of the Serb protest leaders. "We could not wait any longer."

"We have nothing against international judges," said Nebojsa Jovic, another protest leader. "We want to cooperate with all non-Albanians and all those who do not recognize independent Kosovo."

During earlier protests outside the court, U.N. and local staff were forced to evacuate after Serb rioters targeted the building with several small hand grenades.

Kosovo Serbs have already tried to take control of a stretch of rail line in northern Kosovo in defiance of Kosovo's government. Hundreds of Serb policemen have handed over their badges and weapons rather than submit to Kosovo authorities.

Kosovo, which is predominantly ethnic Albanian, has been under U.N. control since 1999, when NATO launched an air war to stop Slobodan Milosevic's crackdown on ethnic Albanian separatists.

Serbia, which considers the territory its historic and religious heartland, says Kosovo's declaration of independence was illegal under international law.