The group, a coalition led by Tjadic's Democratic Party, had 39 percent of the vote, The New York Times reported. Under Serbian law, Tjadic now has 90 days to form a government and will lose that right to the far-right nationalist Radical Party if he fails.
Tjadic said he expects to form a government within the deadline. His supporters, celebrating in Republic Square in Belgrade, waved E.U. flags after the votes came in.
"The citizens of Serbia have chosen the European path, and Serbia will be in the E.U.," he said. "We promise that we will make it."
The Radicals won 29 percent of the votes, while the nationalist party headed by Prime Minister Vojislav Kostunica won 11 percent.