An exit poll by Ipsos for broadcasters TVN, TVP and Polsat on Sunday showed Trzaskowski of the ruling centrists Civic Coalition (KO) winning 50.3 per cent of ballots. His rival, a conservative historian and amateur boxer, Karol Nawrocki, backed by nationalists Law and Justice (PiS), was at 49.7 per cent.
Official results were due on Monday, although a late poll that mixes some results with exit surveys was expected to be published overnight. The exit poll carries a margin of error of two percentage points.
Trzaskowski, 53, campaigned on a promise to help the government of Prime Minister Donald Tusk complete its democratic reforms, which they both say aim to repair an erosion of checks and balances under the previous nationalist government that lost power in 2023.
"We won," he told jubilant party members after the exit surveys came out.
"I will bring people together, I will be constructive, I will be a president for all Poles. I will be your president."
Parliament holds most of the power in Poland, but the president can veto legislation, so the vote is being watched closely in neighbouring Ukraine, as well as in Russia, the US and across the European Union.
Both candidates agreed on the need to spend heavily on defence, as US President Donald Trump is demanding from Europe, and to continue supporting Ukraine in its fight against Russia's three-year-old invasion.
But while Trzaskowski sees Ukraine's future membership of NATO as essential for Poland's security, Nawrocki said recently that if he were president he would not ratify it because of the danger of the alliance being drawn into war with Russia.
Nawrocki, who draws inspiration from Trump and his Make America Great Again (MAGA) movement, said it was too early to call Trzaskowski's victory.
"We will win and we will save Poland," he said.
"We will not allow for Donald Tusk's power to be all-encompassing and the monopoly of evil power ... which takes away our great dreams ...to become complete."
Coming around a year and a half since Tusk, a former president of the European Council, took office, the vote provides the toughest test yet of support for his broad coalition government, with Nawrocki presenting the ballot as a referendum on its actions.
