Hardline nationalist Tomislav Nikolic has won the first round of Serbia's presidential election, but will face a run-off poll, officials say.
Mr Nikolic (in picture) lost to Mr Tadic in the 2004 run-off
Mr Nikolic, who leans towards Russia, appeared to have won 39.6%, while the more pro-EU president, Boris Tadic, had 35.5%, the electoral commission said.
Mr Nikolic said his Radical Party could not be stopped now. Mr Tadic said he was certain of victory in the run-off.
The poll is seen as key for Serbia's future, with tensions high over Kosovo.
One of the main issues facing the new president will be Serbia's breakaway province, whose majority ethnic Albanian population is expected to declare independence in the near future, the BBC's Nick Hawton in Belgrade says.
'Road to victory'
Posters were already being put up in the Serbian capital Belgrade for the 3 February run-off campaign, after it became clear that neither candidate had achieved the 50% necessary to win outright in the first round.
Mr Nikolic hailed his campaign's success and the "extraordinary turnout" of more than 60%.
"Citizens have responded to their wish for change. We opened a road to victory in the run-off. We have never been closer to changes. I want to unite Serbia," he said.
Meanwhile, Mr Tadic said: "It is with a great deal of optimism that I am looking forward to that second round."
He urged Serbians to "turn out to vote in the elections again, that those who procrastinated recently... take part in these elections and show that Serbia absolutely does not give up its European path."
More than 100,000 people voted in UN-administered Kosovo, where voting was organised only in Serb-held municipalities. Ethnic Albanians boycotted the poll.
On Monday, the EU said it was confident that "Serbia will continue to pursue its European course".
"We are convinced that progress towards the EU can be accelerated," the office of EU foreign policy chief Javier Solana said in a statement.
There was a lot of excitement at the headquarters of Mr Nikolic's Serbian Radicals, the country's largest political party, as the early results came in, the BBC's Nick Thorpe reports from Belgrade.
Mr Tadic says EU membership will mean a better life for Serbians
Plates of meat and glasses of brandy appeared in the corridors as news spread that Mr Nikolic was four or five points ahead.
Our correspondent says it appears that the Serbian Radicals have overturned analysts' previously held view that they would struggle to extend their support beyond an existing hard core of voters.
But he adds that Mr Nikolic was beaten by Mr Tadic in the 2004 run-off after leading in the first round.
HAVE YOUR SAY
The EU has been so patronising to us lately that I sympathise with my compatriots who are willing to take their chances with Russia
Milos Milosev, Belgrade, Serbia
Mr Nikolic is standing in for the Radical Party's official leader, Vojislav Seselj, who is still on trial at the UN war crimes tribunal in The Hague.
The campaign was dominated by the issues of Kosovo, the economy and closer ties with the EU.
Mr Nikolic favours eventual membership of the EU but says friendship with Serbia's fellow Orthodox Slavs in Russia is more important.
Serbia's president is the commander-in-chief of the armed forces.
This could be critical in the coming weeks and months, when ethnic Albanians in Kosovo are expected to declare independence, our correspondent says.
Both Mr Tadic and Mr Nikolic are strongly opposed to this.
But Mr Nikolic has projected a more hardline stance on ties with the EU if the bloc recognises Kosovo's independence.
His campaign has benefited from growing frustration at Western backing for Kosovo independence, as well as at the pace of Western-style reforms.
More than 3,000 monitors from Serbia's non-governmental Centre For Free Elections and Democracy (CESID) watched the poll.
However, Serbia's electoral commission had barred US and British poll monitors over their support for Kosovo's independence.