The ultra-nationalist Serbian Radical Party (SRS) has become the biggest single political force in Serbia, after parliamentary elections on Sunday.
SRS supporters expressed jubilation in Belgrade over the weekend
But the party, whose leader Vojislav Seselj is held by the UN war crimes tribunal in The Hague, is not strong enough to form a government by itself.
Four pro-reform groups are now likely to form a governing coalition.
However, correspondents say the new government may be fragile, ineffective and short-lived.
An outgoing deputy prime minister, Nebojsa Covic, said voters had punished the reformist government for failing to raise living standards.
Preliminary results suggest the SRS has won about 28% of the popular vote, but it is set to take nearly one-third of the parliamentary seats.
Slobodan Milosevic's Socialist Party also won 22 seats.
Together they would be able to stop reformers putting together the two-thirds majority required for constitutional changes.
The pro-reform parties are expected to control 147 seats in the 250-seat parliament.
SRS deputy leader, Tomislav Nikolic, toasted the success with champagne and dedicated it to those awaiting trial at the United Nations tribunal for crimes allegedly committed during the wars that led to the break-up of Yugoslavia.
He said the win was also "for the citizens of Serbia who have had enough of humiliation, who want jobs, peace and security, who want to raise their children in a patriotic spirit, and who want a well-rounded state which will not rob anyone of anything but will not give anything to anyone either".
Mr Seselj and Mr Milosevic will not be able to represent their parties in parliament, and it is unclear whether they will be formally allocated seats.
A party official, Ivica Dacic, said: "Technically speaking Milosevic can be a deputy, but our party is yet to decide who is going to take up the seats."
Trouble for reformists
The Organisation for Security and Co-operation in Europe's representative in Serbia, Maurizio Massari, said he was "disappointed" by the result but doubted whether the success of Mr Seselj and Mr Milosevic would have "real impact" on Serbian political life.
The second most popular party, according to preliminary results, 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.
Mr Kostunica said as he cast his ballot that he expected the elections to bring political calm to Serbia.
However, diplomats and correspondents have speculated that there could be new elections within the year.
A deputy prime minister in the outgoing government, Zarko Korac, told the BBC World Service's Newshour programme that it was now "a necessity" for the four pro-reform parties to work together.
However, he added that past experience showed it was hard to build coalitions with the leader of the fourth-placed Serbian Renewal Movement - New Serbia group, the monarchist Vuk Draskovic.
"The question hovering above all of us is how effective the government can be," he said.
Djindjic's Democratic Party appears to have improved its pre-election standing slightly to come in third place.
The surge in support for the ultra-nationalist SRS is being blamed partly on the poor state of the Serbian economy.
The party promised to improve wages and pensions, and to cut the cost of living - all popular policies with an impoverished electorate.