Police were not guarding the embassy at the time
Protesters broke into the US compound in Belgrade and briefly set part of the embassy alight. Firemen later found an unidentified charred body inside.
Other embassies, including the UK's, were also targeted. The UN Security Council condemned the attacks.
The violence followed a peaceful rally by at least 150,000 people in the city.
Most Serbs regard Kosovo as their religious and cultural heartland.
Serbian Prime Minister Vojislav Kostunica delivered an impassioned speech condemning the territory's secession.
[Violence] only keeps Kosovo distant from Serbia,
President Boris Tadic
In pictures: Belgrade rally
"As long as we live, Kosovo is Serbia. Kosovo belongs to the Serbian people," he told the flag-waving crowd.
Later about 1,000 protesters smashed their way into the US embassy, throwing flares through the window while others scaled walls to rip down the US flag.
At the time there appeared to be no police protecting the embassy, but riot police later intervened, firing tear gas.
'Mob attacks'
The fires raged for half an hour, and when firemen finally managed to get inside the building they found a charred body.
The main rally outside parliament was peaceful
White House spokesman Dana Perino said the embassy had been "attacked by thugs" and that Serbian police had not done enough to stop them.
State Department spokesman Sean McCormack said the US had warned Mr Kostunica and his foreign minister that it would hold them personally responsible for further damage.
Mr McCormack added that the protesters had entered the chancellery but had not breached the embassy's secure area.
Smaller groups later targeted the Croatian, Turkish and British embassies but were beaten back.
In New York, the UN Security Council condemned what it called "mob attacks" on US and other embassies in Belgrade.
In a unanimous statement, the council recalled the inviolability of diplomatic missions under international law, but welcomed steps by Serbian authorities to restore order.
Kosovo 'stolen'
Serbian President Boris Tadic appealed for calm.
Serbia, supported by Russia and China, says Kosovo's Sunday declaration violates international law.
During Thursday's rally, ultra-nationalist leader Tomislav Nikolic accused the US and EU of trying to steal Kosovo.
"Hitler could not take it away from us, and neither will today's [Western powers]."
After the speeches, the crowd marched to the city's biggest church, the Temple of Saint Sava.
Thick, black smoke had also earlier billowed from the crossing point at Merdare, 50km (30 miles) north-east of Kosovo's capital Pristina, as Serb army reservists tried to enter Kosovo.
KOSOVO PROFILE Population about two million Majority ethnic Albanian; 10% Serb Under UN control since Nato drove out Serb forces in 1999 2,000-strong EU staff to take over from UN after independence Recognised by US, UK, Germany, Italy and France Not recognised by Russia, Spain, Slovakia, Cyprus Nato to stay to provide security
Country profile: Kosovo Tense confusion on border
"We are here in support of the Serbs who still live in Kosovo," Dejan Milosevic, one of the organisers, told the Associated Press news agency.
The Kosovo police, backed by Czech troops from the Nato-led peacekeeping force, put a steel barrier across the road and were able to hold their line.
Protest rallies were also held in the Bosnian Serb republic (Republika Srpska).
There were unconfirmed reports of injuries as several hundred protesters clashed with police outside the US consulate in Banja Luka.
In the coming weeks, an almost 2,000-strong EU mission will be deployed to help Kosovo develop its police force and judiciary.
In Serbia's capital Belgrade protesters have broken into the United States embassy, and have set fire to an office, which is now extinguished. The break-in followed massive protests against Kosovo's independence that was declared last Sunday. A couple of hundreds of thousands protested in front of the parliament building in Belgrade when masked attackers broke into the building and tried to throw office-furniture out the windows. Estimations of a number of protesters vary between 150.000 and 2 millions.

Around 18:00, after the relay, a couple hundred rioters went to Kneza Milosa Street where the US embassy is located. At 18:15 they demolished a part of the embassy and burned it. They also attacked the Croatian embassy, which is around 100 meters from the US embassy on the same street. Around 19:00 police came and clashed with the rioters using tear gas. Riots were all over downtown Belgrade. As of 22:00 the situation was under control but there are still some riots in other streets.

The tear gas polluted a couple blocks, about 350 meters up to Vračar hill.

The nearby Croatian embassy was also attacked. Rocks were thrown at the Canadian embassy building. This could be due to the fact that Canada has not said yet if it recognises Kosovo. Embassies of Turkey, Bosnia and Herzegovina, Belgium, United Kingdom and Germany were also attacked.

One person was found dead at the ground floor of US embassy, around 150 people were injured, including 35 policemen. Around 100 rioters were arrested.

Dirk-Jan Visser, a photo-reporter for the Dutch newspaper ''NRC Handelsblad'' was attacked by rioters. People helped him escape, and he has been taken into hospital with broken bones. He is expected to be kept in hospital until the next day. Ambulances and medical cars were called to the scene to help injured people and protesters, some cars were attacked.

Andrey Fyodorov and Andrey Pavlov, journalists of ''Russia Today'', were also heavily attacked during the riots.

Two McDonald's restaurants on squares Terazije and Slavija were attacked. The restaurant on Slavija has been heavily damaged. Kiosks, stores and banks were robbed all over the centre of Belgrade. The protesters tried to attack radio/television station ''B92'' but police had the scene under control.

"As long as we live, Kosovo is Serbia," Prime Minister Vojislav Kostunica told the crowd from a stage in front of the old Yugoslav parliament building in Belgrade, "We're not alone in our fight. President Putin is with us". A huge banner reading "Kosovo is Serbia" draped the front of the building.

In Washington, State Department spokesman Sean McCormack called out to the Serbian government to protect the U.S. Embassy. He said the U.S. ambassador was at his home and was in contact with U.S. officials.

The United States was one of the first countries, with the United Kingdom, France and Germany to recognize Kosovo as an independent state. Serbia however regards Kosovo as a province and is backed up in this by Russia, China and numerous other countries, including some European Union member states. Kosovo is 90% ethnic Albanian, with in the north a minority of ethnic Serbians.  Belgrade has, however, not been in control over the Kosovo area since 1999, when United Nations took control.

High representative of the Serbian Radical Party, Aleksandar Vucic, said that "those who provoked Serbian people are equally responsible for destruction as rioters are." President of Serbia Boris Tadic and President of the National Assembly Oliver Dulic and other ministers called on peace.