Serbia's president rejected Kosovo's declaration of independence Sunday, calling it a "unilateral and illegal" move and vowing to retake the territory Serbs consider their historic heartland.

Riot police fired rubber bullets and tear gas and staged repeated baton charges in an effort to disperse hundreds of ultra-nationalist rioters who fought a series of skirmishes with the security forces in downtown Belgrade. Eighteen people, including several policemen, were injured, hospital officials said.

"Serbia will ... do everything in its power to revoke the unilateral and illegal declaration of independence," President Boris Tadic said in the capital moments after Kosovo's parliament voted in Pristina to break off from Serbia.

PHOTOS

But he said Serbia would not use force to reclaim the breakaway province, and urged Serbia's political parties and the 130,000 Serbs living in Kosovo "to remain calm."

Prime Minister Vojislav Kostunica lashed out at U.S. President George W. Bush for supporting Kosovo's independence bid, saying the U.S. leader's name would go down in "black letters" in Serbian history.

In Kosovo, Serbs in Kosovska Mitrovica defiantly flew the Serbian flag as they tried to ignore ethnic Albanians' raucous celebrations just hundreds of meters away.

The Serbian government — which steadfastly opposed full statehood for Kosovo during internationally mediated talks last year — said it has a secret "action plan" for Kosovo. It says the plan does not include military action, though officials have said Serbia will seek to retain control of areas in northern Kosovo inhabited by Serbs.

Serbia also warned that it will downgrade relations with any foreign government that recognizes Kosovo's independence. Tadic urged international organizations "to immediately annul this act, which violates the basic principles of international law."

The U.S., Britain, France and Germany are expected to quickly recognize Kosovo's independence, with some EU nations following suit. But Russia and others fear the move sets a dangerous precedent for separatist movements elsewhere.

Moscow on Sunday denounced Kosovo's independence declaration and called for an emergency meeting of the U.N. Security Council. Tadic will attend the emergency session in New York, his office said.

Kostunica criticized Washington for leading international support of Kosovo's bid. The United States also led the 1999 NATO bombing campaign that ended Serbia's rule over the predominantly ethnic Albanian province.

"America places the policy of force above the U.N. Charter. It is violating international legal norms in order to further its own military interests," he said in a televised address to the nation.

"This unprecedented case of lawlessness was brought about by the destructive, brutal and immoral policies of force imposed by the United States," he said. "Millions of Serbs already are thinking of the day of (Kosovo's) freedom which must come."

Kostunica said the government will organize peaceful protests against the declaration.

Hundreds of protesters — described by police as soccer hooligans — rallied outside the U.S. Embassy in Belgrade. Skirmishes broke out after riot police moved in to disperse them, and groups of masked thugs ran through downtown Belgrade, at one point breaking the windows at a McDonalds restaurant.

They later stoned the embassy of Slovenia — which holds the European Union's rotating presidency — breaking windows and tearing down Slovenian and EU flags.

Members of the elite gendarmerie unit fired tear gas canisters and rubber bullets at the rioters, who broke into tobacco stands and ransacked them.