Ultra-nationalists have emerged as the biggest force in Serbia's general election with 27.5% of the vote, independent election monitors say.
Turnout approached 60%, election monitors said
The Serbian Radical Party (SRS) is led by Vojislav Seselj, a war crimes suspect behind bars in The Hague.
Three main pro-democracy parties together are set to win 41.5%, indicating that together they would be able to prevent the SRS taking power.
The election is seen as a watershed for Serbia's fledgling democracy.
The turnout was just over 59%, or 3.8 million voters, according to the monitors.
This would make it the highest since the Socialist Party of former Yugoslav President Slobodan Milosevic was defeated in 2000.
But in a strange twist, it seems almost certain that Mr Milosevic - who is himself standing trial in The Hague - will be elected to the parliament.
He tops the list of the Socialist Party, which is set to clear the 5% threshold for seats in parliament.
Leaders of the former democratic bloc which ousted Mr Milosevic promised a stable new government, despite being deeply split.
But the high turnout seems to indicate that many Serbs opposed to the Radicals have been voting in greater numbers than expected.
It seems likely that democrats will form a coalition after Sunday's poll.
Two or three parties are expected to work together in a broadly pro-Western European bloc.
But they have warned that it will be hard to find a consensus to push forward stalled reforms.
The second most popular party, according to polls, is the Democratic Party of Serbia led by Vojislav Kostunica, who replaced Mr Milosevic as Yugoslav president in 2000.
Mr Kostunica has been at odds with supporters of former Prime Minister Zoran Djindjic, who was assassinated in March, since soon after Mr Milosevic's overthrow.
But Mr Kostunica said as he cast his ballot that he expected the elections to bring political calm to Serbia.
"I'm expecting that the elections will unblock institutions,
bring political stability to Serbia ... speed up our European
integration and the European future of Serbia," he said.
Mr Djindjic's Democratic Party was trailing in fourth place in pre-election opinion polls.
The surge in support for the ultra-nationalist SRS is being blamed on the poor state of the Serbian economy.
SRS deputy leader Nikolic: "Long live Great Serbia"
The party is promising to improve wages and pensions, and to cut the cost of living - all popular policies with an impoverished electorate.
The BBC's Matthew Price in Belgrade says some Serbians are genuinely worried about a return to nationalism, but to allay their fears Radical deputy leader Tomislav Nikolic says the days of war between nations here are long gone.
However, he admits relations with neighbouring countries would probably cool if he was in power.