With 98.68% of votes counted Tomislav Nikolic, deputy chairman of Serbia's ultranationalist Radical Party, won 39.96% in the first round. Pro-Western incumbent leader Boris Tadic received 35.41%, and other candidates garnered less than 10%.
According to Serbian law, if no candidate gains more than 50%, the leading two contenders must compete in a runoff, which has been scheduled for February 3.
The third and fourth-placed candidates in the first voting round share Nikolic's hard-line stance on many key foreign policy issues, putting him in a strong position for the runoff, according to analysts.
Both Tadic and Nikolic staunchly oppose independence for Serbia's breakaway province of Kosovo.
In the first round a total of 1.91% of ballot papers were declared invalid. Voter turnout was around 61.5%, higher than in recent elections. The final results of the first round will be announced by 7 p.m. GMT today.