The centre, whose representatives observed vote tallying at polling stations across Serbia, said the Radicals were running a distant second with 28.6 per cent, and that the conservative coalition of Vojislav Kostunica, the prime minister, had about 11.6 per cent.
It said that the Socialist Party, loyal to Slobodan Milosevic, the late deposed Serbian president, had about 8.2 per cent.
Tadic's opponents acknowledged that their own vote tabulations confirmed the victory of his pro-Western Coalition for a European Serbia.
The bloc's strong showing came just three months after protesters, outraged by Kosovo's February 17 independence declaration, set fire to part of the US embassy in Belgrade.
The result was a dramatic turnabout after weeks of speculation that the Radicals and Kostunica's coalition together would sweep to victory.
The results instead left Kostunica fighting for his political future.
Tadic said: "This is a great day for Serbia. The citizens of Serbia have confirmed Serbia's European path.
"Serbia will be in the European Union. We have promised that, and we will fulfill that."
He was addressing thousands of his supporters as they waved party and EU flags, honked horns and played the Rolling Stones' Satisfaction song over loudspeakers in central Belgrade.
Tadic acknowledged his rivals could still team up against his coalition and try to form the next government.
Omar Khalifa, Al Jazeera's correspondent in Belgrade, said that despite their victory claim, the pro-EU parties are in some difficulty.
|Tadic supporters took to the streets to |
celebrate victory late on Sunday
Any alliance that can muster a simple 126-seat majority in the 250-seat parliament can govern, and Serbian nationalists indicated they would mount a challenge.
Although Tadic's coalition appeared assured of 103 seats, the Radicals were poised to get 76 seats.
If they joined forces with the Kostunica's coalition, with 30 seats, and the Socialist Party, with 21, the combined strength would be 127 seats.
"Kostunica's bloc, with the Radical Party and the Socialist Party, can realistically combine to win enough seats to form a majority governing coalition," our correspondent said.
"Despite the victory celebrations of Tadic's party, the next government may very well swing in favour of the nationalists, with the unexpected kingmaker of the coming coalition being the Socialist Party."
After the trends became clear on Sunday, Tomislav Nikolic, the Radicals' leader, urged his allies to pull together, and said he would meet Kostunica on Monday and the Socialists to form a government.
Nikolic also accused Tadic of inciting violence by proclaiming victory.
But Tadic made it clear that he saw Sunday's outcome as a mandate to take the divided country into the EU.
"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," he told supporters.
Tadic said he would propose a new prime minister from his own bloc.
"I'm getting ready for tough negotiations on the next government ... those talks will not be easy," he said.