BELGRADE, Serbia-Serbia's rival pro-Western and nationalist camps each geared up for talks on forming a new government Monday after parliamentary elections left the Balkan nation sharply divided.

A nationalist challenge cast a shadow over pro-Western President Boris Tadic's claim of victory in Sunday's vote. The showdown reflected deep divisions among Serbs torn over whether to join the European Union or shift toward their traditional ally, Russia, and revert to their nationalist past.

Tadic proclaimed "a great day for Serbia" after projections by an independent monitoring group and partial results from the state electoral commission gave his Coalition for a European Serbia a 10 percentage-point lead over the ultranationalist Radical Party.

But he told supporters early Monday in central Belgrade: "You should celebrate, but I must go and negotiate."

"Those will be tough negotiations," Tadic said.

His nationalist opponents, meanwhile, sought to team up and form a government despite the pro-Western camp's clear lead.

"There is a clear chance that a government will be formed that will not include Tadic's party," ultranationalist leader Tomislav Nikolic warned.

Nikolic said he would meet Monday with Prime Minister Vojislav Kostunica's conservative coalition and with the late Serbian strongman Slobodan Milosevic's Socialists, whose support looked to be decisive.

Any alliance that can muster a simple 126-seat majority in the 250-seat parliament can govern. Although Tadic's coalition appeared assured of 103 seats, Nikolic's Radicals were poised to get 76. If they joined forces with Kostunica's bloc and the Socialists, their combined strength would be 127 seats.

Nikolic also accused Tadic of inciting violence by proclaiming victory. But Tadic made clear he saw the outcome as a mandate to take the country into the EU.

He warned his opponents "not to tamper with the will of the people" and pledged to prevent the formation of a nationalist government.

Tadic also was expected to court the Socialists and their 21 seats.

The European Union called the success of Tadic's coalition a "clear victory" by pro-European forces.

Swedish Foreign Minister Carl Bildt said pro-European parties have won an "important moral victory." But he warned: "There is no altogether unambiguous majority. The maneuvering can take a great deal of time."

Official results were not expected until later Monday, but the state electoral commission issued partial results that corresponded to projections by the Center for Free Elections and Democracy and tabulations by the main parties.

The respected center, whose representatives observed vote tallying at polling stations across Serbia, said Tadic's bloc had 39 percent. It said the Radicals ran a distant second with 28.6 percent, and Kostunica's bloc had about 11.6 percent. The Socialists had about 8.2 percent, their best result since Milosevic's ouster in 2000.

The pro-Western coalition's surprisingly strong showing came just three months after protesters outraged by Kosovo's independence declaration set fire to part of the U.S. Embassy in Belgrade.

That anger had stoked expectations of an electoral backlash and a Radical victory that would have trampled Serbia's efforts to prepare for eventual EU membership. But analysts in Serbia said Monday that voters apparently were more concerned about lifting living standards than nursing bruised national pride over the loss of Kosovo.

"The success of the pro-European forces has shown that the wish for a better life has prevailed over the anger over the loss of territory," the conservative Politika daily wrote.

Tadic has been publicly denounced as a traitor for signing a pre-entry aid and trade pact with the EU, a deal that Kostunica and Nikolic contend amounts to blood money in exchange for giving up Kosovo.

Milosevic was ousted by a pro-democracy movement in 2000. The former leader, who presided over the bloody 1990s breakup of Yugoslavia, died in March 2006 in a prison cell in The Hague, Netherlands, where a U.N. tribunal was trying him for atrocities in the Balkans.