Colombians are voting in a run-off election to choose a successor to outgoing President Alvaro Uribe.
Juan Manuel Santos, the favourite and an ex-defence minister, has backed Mr Uribe's tough line on security and nearly won outright on 30 May.
His opponent, Antanas Mockus, a former mayor of Bogota, wants to end the corruption scandals that have swirled around the Uribe government.
Security is tight with some 350,000 police and soldiers on duty.
The sale of alcohol has been been banned for the day and the country's borders and ports are closed.
The BBC's Jeremy McDermott, in Medellin, says all opinion polls suggest that Mr Santos will be Colombia's next president - the question is by what margin.
Mr Santos, standing for Mr Uribe's Social National Unity Party, has portrayed himself as the heir to the outgoing president, whose approval ratings remain as high as 60% even after eight years in office.
Mr Uribe had hoped to seek a third term himself, but proposals to change the country's constitution to allow him to stand again were ruled illegal by Colombia's Constitutional Court.
Instead, Mr Uribe's supporters have transferred their backing to Mr Santos.
There has been a stampede by pro-government and even opposition politicians to align themselves with the former defence minister, our correspondent says, since he emerged more than 24 points ahead of Mr Mockus, of the Green Party, in the first round.
But Mr Santos failed to reach the 50% threshold for an outright win and is now required to face Mr Mockus in a second round of voting.