BELGRADE (Reuters) - Serb protesters ransacked and set fire to the U.S. embassy in Belgrade on Thursday, venting anger at U.S. support for Kosovo’s declaration of independence.

A charred body was later found inside, but all American personnel had been accounted for and nothing suggested it was an embassy employee, U.S. officials said.

Riot police -- nowhere to be seen when the attack began -- eventually moved in to disperse the rampaging protesters but Washington was furious.

At U.S. request, the U.N. Security Council condemned “mob attacks” on the embassy in Belgrade.

U.S. Undersecretary of State Nicholas Burns called Serbian Prime Minister Vojislav Kostunica and Foreign Minister Vuk Jeremic to protest at the lack of security.

“The message was very clear, that the situation was intolerable, that they needed to act immediately to provide adequate security,” a State Department spokesman said.

Kostunica, who had earlier addressed a state-backed rally by some 200,000 Serbs against Kosovo’s secession, had promised it would not happen again.

There were lesser attacks on other diplomatic missions, but none was entered. Germany said its embassy was attacked, Croatia protested over damage and local agencies said missions of Britain, Turkey and Bosnia were also hit.

Hospital officials said around 150 people were injured in street clashes, including 30 police and a Dutch reporter.

Serbia’s foreign minister swiftly condemned the violence by a few hundred “vandals”.

Jeremic told Reuters in an interview the attacks on embassies and foreign businesses were unacceptable and regrettable acts by isolated extremists.

“They hurt Serbia’s image abroad, they do not represent the collective feeling of the Serbian people,” he said. “Acts of violence are not going to be permitted on the streets of Belgrade or anywhere in Serbia.”

Gangs also vandalized shops and banks, especially Western ones, leaving a trail of smashed glass and debris. There was some looting.

Serbia considers Kosovo its historic heartland and has waged a diplomatic campaign against the secession of the predominantly ethnic Albanian region, which declared independence on Sunday.

Thursday’s “people’s rally” from which the rioters emerged was Serbia’s biggest since protesters filled the streets in 2000 and stormed the old Yugoslav parliament building to oust nationalist autocrat Slobodan Milosevic.

“As long as we live, Kosovo is Serbia,” Kostunica told the crowd in front of the same building.

“We’re not alone in our fight. President (Vladimir) Putin is with us,” he said, paying tribute to the Russian leader who has opposed U.S. and European states’ recognition of Kosovo.

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The rally had been subdued as Serbs of all ages listened to speeches, melancholic patriotic songs and poems about Kosovo, seen as the birthplace of a medieval kingdom and Serbia’s religious roots.

NO POLICE

When police arrived to disperse the crowd at the U.S. embassy -- under attack for the second time in a week -- they fired teargas and beat and detained rioters. They secured the streets and tried to cordon off the whole embassy district.

Rioters -- many wearing balaclavas and scarves to hide their faces -- had attacked the U.S. embassy with sticks and metal bars after destroying two guard boxes outside.

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They ripped metal grilles from windows and tore a handrail off the entrance to use as a battering ram and gain entry.

One man climbed up and ripped the Stars and Stripes off its pole.

Other people jumped up and down on the balcony, holding up a Serbian flag as the crowd below of about 1,000 people cheered them on, shouting “Serbia, Serbia”.

Black smoke billowed out of the embassy. Papers and chairs were thrown out of the windows, with doors wedged in the window frames and burning. American officials said only security personnel were at the embassy at the time, in a different area.

PRAYERS

Meanwhile, the main rally proceeded as planned with a march to the city’s biggest Orthodox cathedral for a prayer service.

State television switched between scenes of the rioting and the serenity of choral singing at the church service.

The lack of passion at the main rally appeared to support comments by Western analysts and some ordinary people that most Serbs were bitter but resigned to the loss of Kosovo and tired of years of conflict with neighboring states.

“I don’t think this protest might change anything, but I don’t see any other way to express my dissatisfaction,” said Danica, a government employee who declined to give her surname.

In other protests, several hundred Serb army veterans stoned Kosovo police at a border before dispersing. No one was hurt.

In Banja Luka in Bosnia, several people were hurt when protesters clashed with police at the U.S. consulate.