Preliminary counts give the coalition, led by the Democratic Party of President Boris Tadic, up to 39 percent of the votes -- a full 10 percentage points ahead of Tomislav Nikolic's Radical Party.
Democratic supporters didn't wait for the official results -- due by the evening of May 15 -- to celebrate the outcome, setting off fireworks and carousing in the streets of Belgrade.
Proclaiming victory overnight to a crowd of jubilant supporters, Tadic said: "This is a great victory for Serbia. This is a great victory for Serbia's democracy. This a great victory for a European future."
The European Union -- which says it will offer a democratic Serbia a premembership pact -- welcomed the results and urged the winners to quickly form a government "with a clear European agenda."
The polls come about three months after Albanian-majority Kosovo unilaterally declared independence from Serbia, prompting the fall of the year-old government of nationalist Prime Minister Vojislav Kostunica.
Political analysts had predicted that anger over EU support for Kosovo would translate into an electoral backlash in favor of nationalist parties.
Ultranationalist Nikolic remained defiant following the May 11 vote, warning democrats against celebrating too soon.
"There is a great chance of a coalition being formed that will not include the Democratic Party," Nikolic said. He added that "citizens of Serbia...are rightly celebrating tonight because their list won the most votes, but Boris Tadic does not have the right to present it as a celebration of those who will form the new government. The government may be formed however they want, but for it to be approved, 126 delegates have to vote in favor."
Popular support is strong for Serbia's three nationalist parties -- Nikolic's Radical Party, Kostunica's conservative bloc, and former Serbian strongman Slobodan Milosevic's Socialists.
Nikolic said the party leaders will meet on May 12 to discuss forming a government. According to preliminary results, the three nationalist parties would have a combined 127-seat majority in parliament.
The Democrats could form a coalition with the small Liberal Democratic Party -- if it passes the required 5 percent threshold needed to enter parliament -- and other minority parties.
Political analysts say another option could be an alliance with the Socialists, seen as closer to Tadic than other nationalist parties.
A coalition with Kostunica is seen as unlikely, as Kostunica himself made clear after casting his ballot in Belgrade.
"Differences between President Tadic's party and us are insurmountable," Kostunica said. "We will not negotiate on our policy. It will not change. Our clearly stated principles with regard to state and national policy are something which we will never give up."
Parliament must convene by mid-June and a government be formed by mid-September, or else the country must hold a new election.