Boris Tadic claimed victory over Tomislav Nikolic in Serbia's presidential election runoff.
The election is seen as a referendum on its future in the EU.
'These elections are over and I can announce that we have won the presidential elections all together,' Mr Tadic said.
'I congratulate all the citizens of Serbia on us being a European democracy''he said.
With 57% of the vote counted, Mr Tadic had 51.16% against 47.18% for Tomislav Nikolic according to preliminary results.
Turnout was the highest in Serbia since late president Slobodan Milosevic was defeated in elections in September 2000.
Voters faced a stark choice between reformist incumbent Boris Tadic, who has promised EU-backed prosperity, and ultra-nationalist Tomislav Nikolic, who wants stronger ties with Russia.
Central to a fiercely fought campaign is Kosovo, a southern province whose majority ethnic Albanians are threatening to make a declaration of independence within days.
Mr Nikolic won January 20's first round with 40% of votes against Mr Tadic's 35%.
