BELGRADE, Serbia — Serbia’s pro-Western president declared victory in yesterday’s parliamentary elections — a stunning upset over ultranationalists who tried to exploit anger over Kosovo’s independence. But his rivals vowed to fight on, and it was unclear if he could stave off their challenge.

“This is a great day for Serbia,” Boris Tadic proclaimed after an independent monitoring group that carried out a parallel vote count nationwide said his party won 39% — about 105 more than the ultranationalist Serbian Radical Party.

“The citizens of Serbia have confirmed Serbia’s European path,” he said. “Serbia will be in the European Union. We have promised that, and we will fulfill that.”

Mr. Tadic, however, acknowledged his nationalist rivals could still team up against his Coalition for a European Serbia and try to form Serbia’s next government. Any alliance that can muster a simple 126-seat majority in the 250-seat parliament can govern, and nationalists indicated they would mount a challenge.

Although Mr. Tadic’s coalition appeared assured of 103 seats, the Radicals were poised to get 76 seats. If they joined forces with Prime Minister Kostunica’s conservative coalition, with 30 seats, and the Socialist Party of the late Serbian strongman Slobodan Milosevic, with 21, the combined strength would be 127 seats.

Radical leader Tomislav Nikolic urged his allies to pull together, and said he would meet today with Mr. Kostunica and the Socialists to form a government, “because ideologically we are very close.”

Mr. Nikolic also accused Mr. Tadic of inciting violence by proclaiming victory. But Mr. Tadic made clear he saw Sunday’s outcome as a mandate to take the divided country into the E.U.

“I’m sure that those who wanted to return Serbia to the 1990s will try to overturn the electoral will of the people, but I will not allow it,” Mr. Tadic told supporters, adding that he would propose a new prime minister from his own bloc.

Mr. Tadic’s opponents said their own vote tabulations confirmed the pro-Western forces’ victory — an astonishing turnabout after weeks of speculation that the Radicals and Mr. Kostunica together would sweep to victory. The results instead left Mr. Kostunica fighting for his political future.

Mr. Kostunica said yesterday evening his differences with Mr. Tadic’s coaliition were “insurmountable,” and that he was open to talks with the Radicals.

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

The respected center, whose representatives observed vote tallying at polling stations across Serbia, said the Radicals were running a distant second with 28.6%, and that Kostunica’s bloc had about 11.6%. It said the Socialists had about 8.2% — their best result since Mr. Milosevic’s ouster in 2000.

The pro-Western coalition’s surprisingly strong showing came just three months after protesters outraged by Kosovo’s February 17 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 squelched Serbia’s efforts to prepare for eventual E.U. membership. The Radicals had vowed to steer the country away from the West and toward Russia, and openly defy international demands for the arrest of General Ratko Mladic and other fugitive war crimes suspects.

The European Union called the success of Mr. Tadic’s coalition a “clear victory” by pro-European forces.

Yesterday’s elections were the first in Serbia since Kosovo declared independence. Many had expected widespread anger to propel the Radicals to victory, and warned that it could plunge the country into fresh isolation.