BELGRADE, Serbia — Serb rioters broke into the U.S. Embassy and set fire to an office Thursday, and police clashed with protesters outside other embassy buildings after a large demonstration against Kosovo’s declaration of independence.

Masked attackers broke into the U.S. compound, which has been closed this week, just after 7 p.m. and tried to throw furniture from an office. They set fire to the office and flames shot up the side of the building.

It took police about 45 minutes to appear at the scene, and firefighters arrived about the same time and put out the blaze. Police secured the U.S. Embassy and surrounding area, blocking off all access.

The U.S. Ambassador to the United Nations, Zalmay Khalilzad, said he was “outraged” by the attack and would ask the U.N. Security Council to issue a unanimous statement “expressing the council’s outrage, condemning the attack, and also reminding the Serb government of its responsibility to protect diplomatic facilities.”

Serbia’s President Boris Tadic, on an official visit to Romania, appealed for calm and urged the protesters to stop the attacks and move away from the streets. Tadic said that violence was “damaging” Serbia’s efforts to defend Kosovo, which declared its independence from Belgrade on Sunday.

More than a dozen nations have recognized Kosovo’s declaration of independence, including the United States, Britain, France and Germany. But the declaration by Kosovo’s ethnic Albanian leadership has been rejected by Serbia’s government and the ethnic Serbians who populate northern Kosovo.

For several days, Kosovo’s Serbs have shown their anger by destroying U.N. and NATO property, setting off small bombs and staging noisy rallies.

On Thursday, the neighboring Croatian Embassy also was targeted by the same group of protesters at the U.S. Embassy, and smaller groups attacked police posts outside the Turkish and British embassies in another part of the city but were beaten back.

Elite police paramilitaries drove armored jeeps down the street outside the U.S. Embassy and fired dozens of tear gas canisters to clear crowds. The protesters fled into side streets where they continued clashing with the police.

Groups also broke into a McDonald’s restaurant and demolished the interior. A number of other shops were also ransacked and people were seen carrying off running shoes, track suits and other sporting goods from a department store.

Doctors at Belgrade’s emergency clinic reported treating more than 30 injured, half of whom were policemen. All were lightly injured, said Dusan Jovanovic, deputy chief of the clinic, adding that most of the injured protesters were “extremely drunk.”

In Washington, State Department spokesman Sean McCormack said the U.S. ambassador to Serbia was at his home and in contact with U.S. officials. Security officials and Marine guards were in a different part of the compound, but nobody was inside the embassy building, he said.

“We want to strongly urge them, and we are in contact with them, to make sure that they devote the assets to deal with this situation,” McCormack told reporters, referring to the Serbian government.

Serbia has “a responsibility now to devote the adequate resources to ensure that that facility is protected,” he said.

Kosovo, which is 90 percent ethnic Albanian, has not been under Belgrade’s control since 1999, when NATO launched airstrikes to halt a Serbian crackdown on ethnic Albanian separatists. A U.N. mission has governed Kosovo since, with more than 16,000 NATO troops and KFOR, a multiethnic force, policing the province.

But Serbia – and Kosovo’s Serbs, who make up less than 10 percent of Kosovo’s population – refuse to give up Kosovo, a territory considered the ancient cradle of Serbs’ state and religion.

Earlier Thursday, police estimated that about 150,000 people had attended a rally in the Serbian capital. The crowd waved Serbian flags and carried signs reading “Stop USA terror.” One group set fire to a red-and-black Albanian flag.

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Associated Press Writer Edith M. Lederer at the United Nations contributed to this report.