This is a run-off election between pro-western incumbent Boris Tadic and nationalist Tomislav Nikolic, the two biggest vote winners in the first round two weeks ago.
Marko Blagojevic of Belgrade's Center for Free Elections says the turnout is on track to exceed the 61 percent total from the first round.
"Belgrade obviously woke up earlier [than the rest of the country] this morning as the Belgrade turnout by 9am was higher than it was two weeks ago, while the turnout in central Serbia is lower than it was two weeks ago," said Blagojevic.
Voting is said to be brisk among minority Serbs in Kosovo, the southern etnic-Albanian majority province that is expected to soon declare its independence.
The main issue in the campaign has been Serbia's future orientation.
Mr. Nikolic, whose party was aligned with Slobodan Milosevic in the 1990s, favors closer ties to Russia, Serbia's main ally in opposing independence for Kosovo. Mr. Tadic similarly opposes Kosovo independence, but favors closer links with the West and rapid moves towards membership in the European Union.
Pollster Blagojevic says a spirited debate between the two candidates at the end of the campaign has ignited even more voter interest.
"People in Serbia are obviously very interested in these elections, more interested than we could have imagined they would be. And the high number who watched the [televised] duel between the candidates Thursday night is one of the reasons for this," said Blagojevic.
While most power in Serbia rests with the prime minister, the constitution gives the president control of the armed forces. With NATO in charge of security in Kosovo since 1999, close cooperation has recently been established between the regional alliance and the Serbian army. Analysts say those ties would likely be diminished if Mr. Nikolic wins the election. Both candidates say Serbia will not intervene militarily in Kosovo in the event of an independence declaration.