With nearly 98% of votes counted, Tadic's "For a European Serbia" alliance gained 38.75% of the votes - which would translate into 102 out of 250 seats - followed by Tomislav Nikolic's Radicals with 29.2% (78 seats), the State Electoral Commission said. The clear vote comes as a surprise as polls just days before the elections showed Tadic's Democratic Party and the nationalists still neck-and-neck, if anything giving the latter a slight edge.
In his victory speech, President Tadic said his bloc had received a strong mandate to continue Serbia's march towards EU membership, but also reiterated that his new potential government would not recognise Kosovo as an independent state.
Despite his convincing victory, Tadic's alliance still falls short of the necessary majority and might be forced to form a coaltion, either with a nationalist party or the Socialist party of former ruler Milosevic.
The Socialists paradoxically have re-emerged as the potential kingmaker and the party that could bring Serbia back to the West after years of political irrelevance following Milosevic's ousting from power in 2000. The party, which supported Milosevic's wars against the West in the 1990s, has been trying to reinvent itself as an advocate of social justice to attract young and often poor or unemployed voters.
Socialist leader Ivica Dacic indicated on Monday that he would open talks with other nationalist parties, but made it clear that negotiations with Tadic were also on the table.
"This is our great comeback on Serbia's political scene," he said.
"The negotiations will not be easy but I warn everyone not to play with the electoral will of the citizens and try to take Serbia back to the isolation of the 1990s," Tadic said in reference to the former Milosevic regime.
The political combinations do not exclude the possibility that Nicolic's nationalist Radical Party, which favours closer ties with Moscow and China, will form a coalition government with other "ideologically close" parties and the Socialists if Tadic's talks fail.
On Monday the Radical Party, which is expected to gain 77 seats, began to woo the Socialist Party, which has 20 seats, and the nationalist party of outgoing prime minister Vojislav Kostunica, which gained 30 seats. Together the three parties would have 127 seats, enough to govern the 250-seat Parliament.
Accusing Tadic of inciting violence by proclaiming victory, Nikolic said Serbia would either have a nationalist government - with some minority parties - or be forced to call new elections.
Tadic countered that he would not allow popular will to be tampered with and also ruled out Kostunica becoming prime minister. But he did say he was "ready to talk with him, like with everyone else".
Kostunica, who led an emotionally charged campaign arguing that Serbia should punish the EU for its support of an independent Kosovo by turning towards Moscow instead, is seen as one of the big losers of the election, which he triggered when dissolving the government in March.
Analysts considered it "political suicide" for Kostunica to try and form a nationalist coalition with the Radicals.
"He has already lost many moderate voters and if he joins with the Radicals, even more would desert him," Srdan Bogosavljevic of the Belgrade-based Strategic Marketing Research Group told the International Herald Tribune.
The surprise victory by Tadic's pro-European forces - in the first election since Kosovo declared independence from Serbia in February - was whole-heartedly welcomed by Serbian liberals and the EU as evidence that Serbs had chosen economic prosperity and political liberalism over the virulent nationalism of the past.
The electoral commission must publish its final official results by Thursday.
