(CNN)  -- Angry demonstrators protesting Kosovo's independence from Serbia attacked the U.S. Embassy in Belgrade on Thursday, throwing rocks, breaking windows and setting fires.

Serbian TV showed someone trying to set fire to the U.S. flag at the embassy, which was closed and unstaffed when the masked protesters attacked.

Riot police fired tear gas at the rioters and lines of armored vehicles were on the streets before the embassy perimeter was secured. A State Department official told CNN "things are under control."

U.S. State Department spokesman Sean McCormack said; "We are in contact with the Serbian government to ensure that they devote the appropriate assets to fulfill their international obligations to help protect diplomatic facilities in this case."

Kosovo declared independence last Sunday and the United States was among the first countries to offer official recognition of its split from Serbia.

Bratislaw Grubacic, chief editor of VIP magazine in Belgrade, said police reported 32 people injured, including 14 police officers. Video Watch as a protester tries to set fire to the embassy flag »

Teresa Gould, a translator for Belgrade TV, said the Croatian Embassy next door also was attacked. Police quickly rounded up the demonstrators, witnesses said. Video Watch as witness says protestesters threw rocks, Molotov cocktail »

Smaller groups attacked police posts outside the Turkish and British embassies in another part of the city but were beaten back, The Associated Press reported.

The violence was part of a much bigger, peaceful demonstration where up to 150,000 people chanted 'Kosovo is Serbia," and vowed to never accept the province's independence.


Serbian Prime Minister Vojislav Kostunica, who earlier addressed the peaceful rally, said "Kosovo is Serbia's first name." He called the declaration of independence last Sunday illegal and said will do all he can to get it annulled.

State railroads provided free transport to protesters, and schools across the country were closed Thursday for the "Kosovo is Ours" rally in the Serb capital, The Associated Press reported. Video Watch protesters attack the U.S. embassy »

Tensions also erupted at the Kosovo border checkpoint in Merdare -- about 50 kilometers (30 miles) northeast of Kosovo's capital Pristina -- as several hundred Serbian army reservists clashed with NATO-led peacekeepers and police, AP said.

Photographs showed demonstrators, many of them wearing their reservist uniform, hurling rocks and setting tires alight to create a wall of smoke.

U.N. police said the demonstrators had come by bus from the Serbian town of Kursumlija and were largely army veterans who had fought with the Serbian side in Kosovo's 1998-1999 war, AP reported.

Following the clashes, the demonstrators dismantled and returned to the Serbian side of the checkpoint.

Meanwhile several hundred Bosnian Serbs rallied in the Bosnian city of Banja Luka and in the Sarajevo suburb of Lukavica, AP said.

Students in Lukavica were seen waving Serbian flags and singing Serbian patriotic songs while police in Banja Luka were stopping demonstrators from marching on the U.S. consulate there.

The breakaway region has been recognized by the U.S. and several EU nations including the UK, France and Germany but the government in Belgrade maintains that Kosovo is a part of Serbia.

Amid simmering tensions in northern Kosovo, home to most of the region's Serb minority, there were fears that Thursday's rally could spill over into violence, as was seen at the Merdare border crossing, following attacks by Serb nationalists on western targets in Belgrade including the U.S. embassy earlier this week.

The U.S. Embassy in Belgrade advised American citizens to stay away from the gathering, warning that "businesses and organizations with U.S. affiliations may serve as focal points for these demonstrations."

"We wish to remind American citizens that even demonstrations intended to be peaceful can turn confrontational and possibly escalate into violence. American citizens are therefore urged to avoid the areas of demonstrations, and to exercise caution if within the vicinity of any protests," a statement said.

On Thursday, Italy became the latest European nation to recognize Kosovo's sovereignty, AP reported.
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"The recognition of Kosovo's independence does not take away anything from our closeness to Serbia," Italian Prime Minister Romano Prodi told reporters.

Russia and China continue to oppose Kosovo's declaration of independence while Spain has expressed concern that recognition will give momentum to secessionist movements in other countries, such as the Basques in northern Spain.