Counting is under way after polls closed in a knife-edge presidential election in Serbia, which could determine its relations with Europe.
Counting is being monitored closely as a tight result is expected
Exit polls gave the Western-leaning President Boris Tadic a slight lead over his challenger, Tomislav Nikolic.
A pro-Russian nationalist, Mr Nikolic was beaten by Mr Tadic in a similar run-off in 2004.
The election is taking place amid a looming independence declaration from Kosovo, which both candidates oppose.
As the polls closed at 2000 (1900 GMT), voter turnout was estimated at around 66%, Serbia's highest since the 2000 election that ousted Slobodan Milosevic after more than 10 years in power.
The strong turnout is expected to work in the favour of the incumbent by diluting Mr Nikolic's hardcore of supporters, analysts suggest.
Mr Nikolic headed a field of nine candidates in the first round on 20 January, beating Mr Tadic by 40% to 35.4%.
A simple majority of the vote is required for victory in the run-off, and initial results are expected within hours.
'Agony and decline'
With Kosovo looking set to declare independence within weeks, the BBC's Nick Hawton in Belgrade says many Serbs see the vote as a referendum on their country's future.
Kosovo has been run by the UN since 1999, when a US-led Nato bombing campaign drove out Serb forces accused of a brutal crackdown on ethnic Albanian separatists.
Incumbent President Boris Tadic wants Serbia to be part of the EU
The US and most of the EU members back the independence plan, while Serbia and Russia are strongly opposed to it.
Mr Tadic, a psychology graduate and former defence minister, leads the Democratic Party, which has made pushing for Serbian entry to the European Union a priority.
He supports the free market and democratic reforms, and advocates co-operation with the UN war crimes tribunal in the Hague.
After casting his ballot at a Belgrade polling station, he told reporters: "I am totally sure that we are going to take the same direction towards the European Union."
Mr Nikolic, who believes in closer ties with Russia, was one of the founders of the hardline nationalist Serbian Radical Party.
Voting in Belgrade, he told reporters: "Without me Serbia has no future.
"It is a country in agony and decline. This is a country of unfulfilled promises and great expectations, like in Charles Dickens."
The Radicals were allies of the ex-president, Slobodan Milosevic, and their leader, Vojislav Seselj, is currently on trial at the Hague for his role during the wars of the 1990s.
Mr Nikolic strongly opposes co-operation with the Hague, wants closer ties with Russia and has expressed strong doubts about Serbia's relations with the European Union.
One voter, Milica Milivojevic, told the BBC: "I want Kosovo in Serbia, where it belongs, someone to stop the corruption, and Tadic is not the man."
But another voter, Nemanja Stevanovi, said: "If Tomislav Nikolic becomes president people will very soon understand what mistake they have done."
While the presidency is a largely ceremonial office, the president is commander in chief of the armed forces, a role which carries strong symbolic weight, our correspondent says.
Ethnic Albanian leaders in Kosovo have hinted that the expected declaration of independence may be delayed, after saying earlier this week it was "an issue of days".
Kosovo is still technically a southern province of Serbia.
EU states have asked Kosovo's leadership to wait until an EU civilian mission can take over from the UN.