(adds details on protests, parliament session, edits)

By Ellie Tzortzi

BELGRADE, Feb 17 (Reuters) - Serbia's leaders pledged peaceful resistance after Kosovo's declaration of independence on Sunday, but angry protesters turned briefly to violence.

Some 2,000 people vented their anger at American backing for the breakaway province in a protest in Belgrade.

"Kosovo is the heart of Serbia," they chanted, ripping up paving stones and tiles to throw at the U.S. embassy and riot police guarding it. Several police and rioters were injured.

Some 500 demonstrators clashed with police in the northern city of Novi Sad. McDonalds restaurants were vandalised in Novi Sad and Belgrade, while in the Kosovo Serb stronghold of Mitrovica, hand grenades were thrown at EU and U.N. buildings.

One exploded causing no major damage.

Local news agencies also reported protests in Banja Luka, capital of the Bosnian Serb Republic whose leaders look to Kosovo for a precedent they could use to secede from Bosnia.

In Serbia, political leaders were united in their anger at Kosovo's declaration but gave out very different signals on how the move would affect Belgrade's ties with the West.

Pro-Western President Boris Tadic called for calm, saying Serbia will "defend its interests and international law, no matter how long it takes".

"Serbia will never recognise the independence of Kosovo," but "go through this peacefully, with dignity", he said.

But nationalist Prime Minister Vojislav Kostunica, whose party is in a shaky coalition with Tadic's, attacked the United States and EU for supporting the secession of a province Serbs see as their religious and historic heartland.

In a televised address to the nation minutes after Kosovo formally severed ties after nine years under U.N. control, he accused the United States of being "ready to violate the international order for its own military interests".

"As long as the Serb people exist, Kosovo will be Serbia," Kostunica said. Kosovo's declaration was "the final act of the policy of force which started with the insane bombing of Serbia and continued with the arrival of NATO troops in Kosovo".

HUGE RALLY

The United States spearheaded the 1999 NATO air war that expelled Serb forces from the province to stop the mass killing of Albanian civilians in a counter-insurgency crackdown.

Serbia's parliament was due to meet on Monday or Tuesday for an urgent session, state television RTS reported, with party leaders agreed to call citizens to mass protests on Thursday.

Russia, Belgrade's strongest ally, called immediate U.N. Security Council talks on Kosovo. Tadic was heading to New York on Sunday to attend a full council session on Monday which has no real chance of reversing Western backing for Kosovo.

Serbian Foreign Minister Vuk Jeremic will go to Brussels on Monday where EU foreign ministers will meet on Kosovo, which hopes most member states will swiftly recognise it.

Tadic favours separating the issue of Kosovo from Serbia's long-delayed EU membership bid. But Kostunica insists Brussels must give up support of Kosovo if it wants Serbia as a member.

The staunchly nationalist Radicals, Serbia's strongest party, called on Kostunica and Tadic to "channel the unrest and anger" of Serbs into a huge rally.

"As long as there are Serbs, we will go on fighting for Kosovo," their leader Tomislav Nikolic said.

The sharpest reaction was from the Serb Orthodox Church in Kosovo, whose leader Bishop Artemije denounced the army for inaction and said Serbia should buy arms from Russia to fight. (additional reporting by Ljilja Cvekic; editing by Richard Meares)

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