Georgi Parvanov is expected to secure a second five-year term, but opinion polls ahead of Sunday's elections suggest that frustration among voters over the speed of reforms and continuing poverty will result in a runoff with Volen Siderov of the xenophobic Attack Party.
Bulgaria is set to join the European Union on January 1; but Siderov has campaigned against entry.
A survey by the state polling agency NPOC on October 19 showed that the 49-year-old former Socialist leader was leading with 50.5 per cent support. Siderov was trailing with just 23 per cent.
In third with 18 per cent was the most popular candidate from Bulgaria's fragmented right-of-centre opposition, Nedelcho Beronov.
"I am confident in our victory on October 22," Parvanov said as he wrapped up his campaign this week. "As head of the state I will be a guarantee for political stability".
Many Bulgarians have criticised Parvanov for doing too little to push the government into stopping corruption and organised crime and raise living standards that - with average wages of $202 a month - will be the EU's lowest.
Despite standing as an independent, Parvanov has close links with the Socialists and is seen as the architect of their ruling coalition.
Voters have been ambivalent about the election and polls this week forecast turnout of around 40 per cent, below the 50 per cent threshold needed to prevent a runoff between the top two placed candidates on October 29.
Siderov has tapped into the discontent by attacking Bulgaria's minorities and pledging to organise referendums on policy issues including Washington's plans to use military bases in Bulgaria and the Balkan state's participation in international organisations.
Polling stations will open at 0300 GMT and close at around 1600 GMT. The first exit polls are expected a few hours later.