Serbian nationalist Tomislav Nikolic will face a run-off with reformer Boris Tadic, the preliminary results of Serbia's presidential elections say.
Nikolic has consistently led in opinion polls
Mr Nikolic of the Serbian Radical Party led with 31.7% of the vote, with Mr Tadic of the Democratic Party coming close second on 27.9%.
This is Serbia's fourth attempt in 18 months to choose a leader, as previous polls failed because of a low turnout.
A second round winner is certain after the minimum turnout law was abolished.
The preliminary results were released by Serbia's state election commission.
The second round on 27 June is expected to be a close run, and Mr Tadic needs all the support of his democratic rivals from other parties, the BBC's Matt Prodger in Belgrade says.
Mr Tadic has already urged "all democratic forces and citizens to unite their votes and support me in the run-off".
If Mr Nikolic is elected, most people expect the current governing coalition to disintegrate, our correspondent adds.
Rise of radicals
Mr Nikolic's party leader, Vojislav Seselj, is being tried in The Hague, accused of ethnic cleansing, torture and murder during the Balkan wars of the 1990s.
The Serbian Radical Party emerged as the single strongest group in parliament after elections last December but was unable to form a majority coalition.
The rise in support for the radicals has been attributed to disillusionment over corruption, poverty and Serbia's international isolation - all of which have continued since former President Slobodan Milosevic was toppled in 2000, our correspondent says.
Mr Tadic's party was led by the former Prime Minister, Zoran Djindjic, before his assassination last year.
The biggest surprise in the first round was the high level of support won by Serbia's wealthiest businessman, Bogoljub Karic, who entered the election late on a ticket of economic modernisation.
Mr Karic came third with 18.7% of the votes.
The presidential candidate of Serbia's ruling coalition government, Dragan Marsicanin, was in fourth place, polling some 13.3% of the votes.