(CNN) -- Serbia's pro-Western President Boris Tadic declared victory Sunday in parliamentary elections, despite a challenge to his bloc from nationalist groups.
"This is a great victory for Serbia," Tadic said Sunday night. "This is a great victory for Serbia's democracy. This is a great victory for Serbia's European future."
Tadic's coalition, the Coalition for European Serbia, held about 38 percent of the vote, for about 103 seats, according to preliminary results from the Belgrade-based Center for Free Elections and Democracy, or CeSID.
Tomislav Nikolic's ultra-nationalist Serbian Radical Party held about 29 percent of the vote, or about 77 seats, CeSID said.
Prime Minister Vojislav Kostunica's Democratic Party of Serbia -- separate from Tadic's Democratic Party, which leads the coalition -- held about 11.3 percent, or about 30 seats, CeSID said.
The Serbian Electoral Committee, or RIK, published similar preliminary results, showing that Tadic's bloc won about 35 percent of the vote, while the Serbian Radical Party held about 28 percent.
CeSID said voter turnout was about 60.7 percent of registered voters. Watch Serbians go to the polls »
The European Union presidency issued a statement saying it welcomed "the clear victory" of "pro-European forces."
"Provided that the necessary conditions are met ... this should enable Serbia to advance further on its EU path, including the candidate status," the statement said.
Nikolic shot back Sunday.
"By derogating the constitution [and proclaiming victory], Boris Tadic has made conditions for the citizens unhappy," Nikolic said, adding that Tadic is pressuring Serbians to accept his unofficial victory.
Nikolic suggested that the nationalist groups could partner against Tadic to pull a simple 126-seat majority in the 250-seat parliament in order to govern Serbia.
He said there is "a great chance" for creating a coalition government that excludes Tadic's Democratic Party. "It takes 126 members of the parliament to elect and approve the government in the Serbian parliament," he said. "No one can keep them from doing that, not even Boris Tadic."
All 250 parliamentary seats were up for grabs in this election, the first since the province of Kosovo declared independence from Serbia in February.
The election has been widely described as the most important in Serbia's recent democratic history, as the parties presented two different paths for the country's future: putting aside the issue of Kosovo's independence to focus instead on EU membership, or opposing Kosovo independence at all costs.
Kosovo's declaration caused the collapse of Serbia's fragile ruling coalition and a political stalemate between pro-European Union and nationalist groups.
For many Serbs, the idea of forging closer links with Western powers that supported Kosovo's breakaway amounts to treachery.
Nationalists such as Nikolic and Kostunica say there can be no accommodation with the West while Kosovo's status as a Serbian province remains in question, and they threatened to align Serbia with Russia if elected.
Tadic, who led the coalition with Kostunica until its collapse, also says Kosovo belongs to Serbia, but says he's not prepared to make that a condition of EU accession. For that, the nationalists labeled Tadic a traitor.
None of the six parties running in the election backs independence for Kosovo, which has massive religious, cultural and historic significance for Serbs.
When Tadic's government signed a pre-membership agreement last week in an initiative to bolster pro-EU feeling ahead of Sunday's vote, the president said he had received death threats accusing him of "betraying the Serb people."
Freshly painted graffiti on Belgrade walls reads: "Tadic Judas."
Many analysts predict months of wrangling before a coalition government emerges.
In a country torn between withdrawal from the West over Kosovo and greater engagement with the wealth and benefits of the European Union, a longer-lasting solution to Serbia's political crisis may be even more elusive.
"Whether it is a nationalist coalition or a pro-Western coalition, it is going to be a troubled coalition," journalist Dejan Anastasijevic told CNN. "It will have a tiny majority in parliament, it will have the other side breathing down its neck, and there will be infighting within the coalition."
CNN.com's Simon Hooper contributed to this report.