The respected monitoring group said the ultranationalist Serbian Radical Party was running a distant second with 28 percent, and that nationalist Prime Minister Vojislav Kostunica's bloc had about 11 percent.
"Tadic's coalition has convincingly won these elections," said Zoran Lucic, the center's spokesman.
But the ballot was clearly hard-fought, and it remained unclear whether Tadic's pro-Western forces would be able to form a new government with minority parties - or whether the Radicals, Kostunica's coalition and the Socialist Party of the late Serbian strongman Slobodan Milosevic would attempt to team up against Tadic.
The Socialists appeared to have made unexpected gains Sunday with about 8 percent and could play a crucial role in the makeup of the next government.
Sunday's elections were the first in Serbia since Kosovo declared independence in February, and many had expected widespread anger to propel the Radicals to victory.
Officials said turnout was about 60 percent - lower than in January's presidential elections, but strong for a parliamentary vote.