Serbians are voting in a presidential runoff election, with pre-vote opinion polls predicting victory for incumbent Boris Tadic over his nationalist challenger Tomislav Nikolic.
More than 6.7 million eligible voters can cast their ballots until 8pm (18:00 GMT) on Sunday with first preliminary results expected during the evening.
Surveys published just before the second round runoff predicted that Tadic, who has steered Serbia to European Union candidate status after years of isolation, would secure a fresh five-year mandate.
Tadic has focused his campaign on further EU integration and economic development, pledging that Belgrade will start EU membership talks before the end of year if voters hand him a third mandate.
"May 20 is going to bring a big democratic decision - whether we will remain a stable country, or one burdened with problems," Tadic told a final pre-poll rally.
'Thieves and liars'
Tadic came just ahead of Nikolic in the May 6 first round of presidential elections, but the nationalist challenger accused the incumbent of fraud.
The claims have been dismissed both by Serbia's prosecutor and electoral officials, but they have cast a shadow over the polls.
Nikolic insists that an investigation into the claims by the state prosecutor was controlled by Tadic's party and has threatened to "personally fight" any attempt at election fraud in the runoff vote.
"Our intention is to fight. What else can we do?" Nikolic's deputy, Aleksandar Vucic, told Serbian state television. "We are running to beat the thieves and liars."
A one-time ally of former Serbian leader Slobodan Milosevic, who launched the country into a succession of nationalist wars against other former Yugoslav states in the 1990s, Nikolic is a recent convert to the European cause.
The 60-year-old has pledged to stay on a pro-European course but said he would not join the bloc at any cost.
"I support a two-door policy, both toward the East and the West," he said in a heated TV debate with Tadic last week, accusing him of favouring only co-operation with the EU and US.
Serbia's nationalists have traditionally sought to align the country with Russia which has strong ties with the country.
Nikolic has also campaigned on populist promises, such as tax rises for the wealthy which would be used to increase pensions and welfare payments.
Tadic has been able to pull ahead since the first round of vote after securing the endorsement of the Socialists - the third biggest party in the Serbian parliament.
Whoever emerges as president faces tough economic challenges in a country where 24 per cent of 7.5 million inhabitants are unemployed, one of the highest jobless rates in Europe.
"Promises of a European future are nice, but I cannot live off nice words," said Vidoje Markovic, 34, who lost his job as a nurse in a state hospital due to budget cuts.
The next president also faces continuing diplomatic issues over Kosovo, the Albanian-majority breakaway province that declared independence in 2008 which Serbia still claims as its own.
The European Union has made continuing talks between Belgrade and Pristina a key condition for further EU integration.