The nationalist Serbian Radical Party (SRS) has a clear lead in the country's elections, early results suggest.
Radical leader Tomislav Nikolic called on the government to resign
An election monitoring group said the Radicals had taken 28.5% of the vote, but they are not expected to find partners to form a governing coalition.
The pro-EU Democratic Party (DS) and Democratic Party of Serbia (DSS) were credited with 22% and 17% respectively.
EU foreign ministers are meeting in Brussels to discuss the bloc's ties with Belgrade and the future of Kosovo.
"The majority voted for forces that are democratic and pro-European," said Javier Solana, the EU's foreign policy chief, as he arrived.
"I hope very much there will be a speedy formation of a government that will be on the line of pro-European forces".
Serbia's electoral commission said turnout was about 62% of the 6.6 million eligible voters.
After the first projections were announced, the SRS candidate for prime minister, Tomislav Nikolic, said: "We have won as we had expected."
"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 called for the resignation of President Boris Tadic and Prime Minister Vojislav Kostunica, who both campaigned for closer ties with the European Union.
The BBC's Jonny Dymond in Belgrade says the SRS will not be part of any new government because there is no major party that will go into coalition with them.
Mr Nikolic's group will once again sit on the sidelines whilst more moderate parties struggle with Serbia's many challenges, our correspondent adds.
His party fought a strongly anti-Western campaign. Its leader, Vojislav Seselj, is on trial for war crimes at the UN tribunal in The Hague.
The SRS ruled former Yugoslavia for part of the 1990s in coalition with the Socialist Party of Serbia (SPS) of the late leader Slobodan Milosevic.
Prime Minister Kostunica, who heads the conservative DSS, brushed away speculation on who will form a coalition with whom.
SERBIA'S MAIN PARTIES
Serbian Radical Party (SRS) - Tomislav Nikolic
Democratic Party (DS) - President Boris Tadic
Democratic Party of Serbia (DSS) - PM Vojislav Kostunica
Socialist Party of Serbia
Liberal Democratic Party
"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 could end up playing kingmaker, analysts say, by striking a deal to remain prime minister with other parties.
A slew of smaller parties won the remainder of the vote, Serbia's Centre for Free Election and Democracy said.
Reformists G17 Plus won 6.8%, the SPS took 5.9%, and the Liberal Democratic party 5.3%.
Four other parties were forecast to win a handful of parliamentary seats.
Mr Tadic, who leads the DS, said he hoped the outcome would produce a government capable of major reforms.
Correspondents say the main issues facing the next government include economic reforms, membership talks with the EU - currently stalled over co-operation with the war crimes tribunal in The Hague - and Kosovo.
The mostly-ethnic Albanian region is legally a province of Serbia but is under UN administration.
The UN's chief envoy for the province is expected next month to recommend some form of independence - something the SRS vehemently opposes.
The other main parties have proposed wide autonomy for Kosovo within Serbia.
This has been the country's first general election since its union with Montenegro was dissolved last year.