Belgrade, Serbia ? Angry Serbs broke into the U.S. Embassy and set fire to an office Thursday night as rioters rampaged through Belgrade’s streets, putting an exclamation point of violence to a day of mass protest against Western support for an independent Kosovo.

At least 150,000 people rallied in Belgrade, waving Serbian flags and signs proclaiming “Stop USA terror,” to denounce the bid by Kosovo’s ethnic Albanian majority to create their own state out of what Serbs consider the ancient heartland of their culture.

The United States strongly criticized the violence and the Serb response. White House spokeswoman Dana Perino said the embassy “was attacked by thugs” and Serb police didn’t do enough to stop it. State Department spokesman Sean McCormack said the U.S. warned Serbian Prime Minister Vojislav Kostunica and Foreign Minister Vuk Jeremic that it would hold them personally responsible for further damage.

Protesters burned American flags, and the mob that attacked the embassy tore down the U.S. flag there. Crowds also ransacked a McDonald’s, looted stores and fought with police in front of other diplomatic compounds in a display of the resentment seething in Serbia over the secession of what has been its southernmost province.

A charred body was found in the U.S. Embassy after the fire was put out, but all staff were accounted for, embassy spokeswoman Rian Harris said. Belgrade’s Pink TV said the body appeared to be that of a rioter.

At the mass rally earlier, Kostunica attacked the U.S. and others for supporting Kosovo’s independence. “Is there any other nation on Earth from whom (the great powers) are demanding that they give up their identity, to give up our brothers in Kosovo?” he asked the crowd.

Coming after smaller outbursts of violence in Belgrade as well as attacks on a United Nations building and police checkpoints in Kosovo, the surge of rioting underlined the determination of Serbs not to give up Kosovo quietly.

The Serbian government has said it won’t resort to military force, but the street violence could be a tactic to slow moves by more countries to follow the U.S., Britain, Germany and France in quickly recognizing Kosovo’s independence.

Russia and China lead the states standing with Serbia, worrying that Kosovo’s example could encourage separatist sentiment elsewhere. The Kremlin has underlined its displeasure by hinting it might back separatists in pro-Western nations such as the former Soviet republic of Georgia.

Serbian officials dismissed violence earlier in the week as “insignificant,” and no police were guarding the U.S. Embassy compound even though it had been targeted previously. American officials said the offices had been closed at midday because of security concerns.

Milorad Veljovic, a top Interior Ministry official, said that security forces had the situation under control and that mobs had been broken up.

Masked men smashed their way inside the compound’s consular building, tore down the U.S. flag and tried to throw furniture from an office. They set fire to the office and flames shot up the side of the building.

State Department officials said no protesters got into the embassy’s main chancery section, a separate area that was manned by a U.S. Marine guard unit and security personnel.

Police arrived about 45 minutes after the blaze broke out, and after the rioters left the building. A half dozen fire trucks also appeared and quickly doused the flames, leaving the front facade and two police guard posts on the sidewalk smoldering.

Officers from an elite paramilitary police unit drove armored jeeps outside the embassy and fired dozens of tear gas canisters to disperse the crowd. Rioters wheeled out large garbage bins in an unsuccessful attempt to block the police vehicles. Thick clouds of tear gas hung in the street as officers chased rioters into nearby side streets.

It was the first attack on a U.S. Embassy since Sept. 12, 2006, when Syrian security guards stopped an attempt to blow up the compound. The last time a mob broke into one was the Iranians’ seizure of the U.S. Embassy on Nov. 4, 1979, taking the American staffers hostage.