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.
Several hundred men broke away from the massive protest, battering gates and burning the U.S. flag - eventually breaking down doors and setting the U.S. Embassy's visa section on fire, reports CBS News correspondent Sheila MacVicar.
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.
State Department spokesman Sean McCormack said Serbian security "wasn't adequate" at the embassy and said the U.S. government warned Serbia's leaders it would hold them personally responsible for further damage.
"The Embassy for the last few days has been well protected by police but the police today appear to be a bit overstressed because there was an extremely large peaceful official protest in front of the federal parliament building earlier in the day," reporter Neil MacDonald in Belgrade tells CBS Radio News.
Coming after smaller outbursts of violence in Belgrade as well as attacks on a U.N. building and police checkpoints in Kosovo, the outburst 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.
The 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 some 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 obscured 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.
Critics of Prime Minister Vojislav Kostunica accused his nationalistic government of tolerating the violence over Kosovo, perhaps as an excuse to crack down on pro-Western forces in the country.
"I cannot tell if the authorities are going to allow this to escalate, and how long they will let this go on, but it is absolutely clear that it is all under their control," said Vesna Pesic from the pro-Western Liberal Democratic Party whose offices also have been attacked.
The same group of protesters that attacked the U.S. Embassy also targeted the neighboring Croatian Embassy. Smaller groups assaulted police posts outside the Turkish and British embassies in another part of the city but were beaten back.
Rioters broke into a McDonald's restaurant and demolished it. Other shops also were ransacked and people were seen carrying off running shoes, track suits and other sporting goods from a department store.
Police chased small violent groups in areas near the national parliament, where the huge protest rally was held earlier in the afternoon.
Police also were guarding the independent B92 television station - viewed by nationalists as pro-Western - as youths started gathering nearby.
Doctors at Belgrade's emergency clinic reported treating 96 injured, 32 of them policemen. All were lightly injured, said Dusan Jovanovic, deputy chief of the clinic. He added that most of the injured protesters were "extremely drunk."
Serbian President Boris Tadic, who was visiting Romania, appealed for calm and urged protesters to get off the streets. He said the violence was "damaging" Serbia's efforts to defend its claim to Kosovo.
The government went all out to make the mass demonstration outside the parliament a success. Schools were closed and the state railway offered free rides to bring people to Belgrade.
"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?" Kostunica, the prime minister, told the crowd.
Tomislav Nikolic, a member of the ultra-nationalist Radical Party, which is Serbia's largest, accused the U.S. and the European Union of stealing Kosovo. Protesters booed and jeered at every mention of the U.S. and EU.
"We will not rest until Kosovo is again under Serbia's control. Hitler could not take it away from us, and neither will today's (Western powers)," Nikolic said.