Poles are voting in a presidential election, seen as a battle for the country’s future and its strained relations with the European Union.
The second round pits incumbent Andrzej Duda, an ally of the conservative government, against socially liberal Warsaw mayor Rafal Trzaskowski.
A Duda win would herald controversial changes to the judiciary and continued opposition to abortion and gay rights.
Mr Trzaskowski backs a more progressive agenda and active role in the EU.
Mr Duda topped the first round of voting with a convincing lead but fell short of the 50% needed to win outright.
But Mr Trzaskowski expects to win the support of most of those who voted for other candidates, and a close result is predicted.
Polls opened at 07:00 local time (05:00 GMT) and close at 21:00.
The BBC’s Adam Easton, reporting from a polling station in Warsaw, says there is a steady stream of people arriving and putting on face masks to vote, but no sign of the long queues seen two weeks ago in the first round, which saw the highest turnout in 25 years.
President Duda is an ally of the governing nationalist Law and Justice (PiS) party and if he loses, the opposition could push through major changes in Polish politics.
The president has the power to veto legislation, so Mr Duda’s re-election would be of benefit to PiS, of which he used to be a member.
He came under fire during the election, including for a speech in which he said LGBT rights were an “ideology” more destructive than communism.
Speaking on the last day of campaigning on Friday, Mr Duda said he would continue strengthening the Polish state, “built on our inviolable tradition which is sacred to all of us and in which we have been brought up for generations”.
Mr Trzaskowski has pledged to heal rifts with the European Union.
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The liberal politician rose fast in the polls after joining the race in May. Previously a member of Donald Tusk’s liberal Civic Platform government, he won the capital’s race for mayor in 2018 promising “Warsaw for All”.
He said Polish voters would never have another chance to change Poland’s direction.
“Everything is in your hands,” he said on Friday. “This [electoral victory] is really at your fingertips. I want to tell you ‘now or never’ because this Law and Justice road roller will try to run over everything and the next election could be very difficult.”