Nationalists 'lead Serbia poll'
Radical leader Tomislav Nikolic called on the government to resign
The nationalist Serbian Radical Party (SRS) has a clear lead in the country's elections, early projections suggest.
The Centre for Free Elections and Democracy, a monitoring body, said the radicals had taken 28.5% of the vote.
The SRS candidate for prime minister, Tomislav Nikolic, urged the government to resign - but it is unclear whether he can form a governing coalition.
The pro-reform Democratic Party (DS) and Democratic Party of Serbia (DSS) were credited with 22% and 17%.
Because no one party has got a clear majority of the vote, a coalition will have to be formed.
Serbia's electoral commission said turnout was about 62% of the 6.6 million eligible voters.
The BBC's Nick Hawton in Belgrade says the relatively high turnout reflects the close contest between Serb nationalists and the more pro-European parties.
After the first projections were announced, Mr Nikolic said: "We have won as we had expected."
The question who is going with whom is inappropriate at this moment
Serbian Prime Minister Vojislav Kostunica
"Despite running against the parties led by the prime minister and the president (Boris Tadic) and their vicious campaigns against us, we proved our strength," Mr Nikolic said.
He ruled out a coalition with the governing DS and DSS but on the subject of coalitions with other parties he said: "Let's wait and see the final results. Things are clearer after a night's sleep."
The SRS ruled the country under late leader Slobodan Milosevic during the 1990s.
Prime Minister Vojislav Kostunica, who heads the conservative DSS, brushed away speculation on who will form a coalition with whom.
"The question who is going with whom is inappropriate at this moment," he said.
President Tadic "will have to find a man who will be able to get a majority" in the parliament, he added.
Mr Kostunica had urged citizens to vote - it was, he said, the best way to keep the country on a safe and stable path.
Mr Tadic, whose Democratic Party is challenging the prime minister, said he hoped the outcome would produce a government capable of major reforms
The poll was overshadowed by the issued of Kosovo, with the UN expected to rule on the future of the province this year.
It was the country's first general election since its union with Montenegro was dissolved last year.