Around 30 million Poles out of a total population of 38 million are eligible to vote.
Turnout in the first round was 54 per cent, but the Komorowski camp fears that the timing of the election, in mid-summer, combined with unusually hot weather, will play to Kaczynski's advantage as Komorowski voters, who are generally younger and wealthier, are more likely to take holidays and not vote.
Still, opinion polls have mostly predicted a Komorowski victory, although Kaczynski has been narrowing the gap in recent weeks and lagged by just five percentage points in the first round of voting on June 20.
A final slew of polls published on Friday, the last day of campaigning, showed the candidates either on level terms or Komorowski with a small lead.
Financial markets favour a Komorowski presidency, expecting him to work smoothly with the market-oriented government of Donald Tusk, the prime minister, as it tries to rein in a big budget deficit while keeping a fragile economic recovery on track.
"Only co-operation can guarantee that money will be spent rationally, only co-operation can guarantee that Poland will take the path of development," Komorowski said on Friday.
In Poland, the government led by the prime minister sets policy, but the president appoints many key officials, has a say in foreign and security policy, and can propose and veto laws.
Investors fear that Kaczynski, who opposes cuts in public spending and privatisation, would use his presidential veto to block reforms, just as his brother Lech did before his death.
However, Kaczynski, known in the past for his acerbic nationalist rhetoric, has struck a conciliatory tone on the campaign trail in an apparent bid to win over middle-of-the-road voters.
"As president I would want to convince people to co-operate for the common good, for the development of Poland," he said on Friday, on a final swing through rural Poland, heartland of his conservative supporters.
Kaczynski is opposed to joining the euro any time soon and is distrustful of the EU, Russia and Germany.
Komorowski, the speaker of parliament and the country's acting president, shares Tusk's vision of a Poland firmly anchored in the European mainstream.
He favours working closely with Germany and other EU partners and trying to improve long troubled ties with Russia.