In a victory for European conservatives inspired by US President Donald Trump, Nawrocki secured 50.89 percent of the vote, election commission data showed, an outcome that presages more political gridlock as he is likely to use his presidential veto to thwart Prime Minister Donald Tusk's liberal policy agenda.

Tusk's government has been seeking to reverse judicial reforms made by the previous nationalist Law and Justice (PiS) government, but current President Andrzej Duda, a PiS ally, has blocked its efforts - a pattern Nawrocki is likely to continue.

Nawrocki's rival, Rafal Trzaskowski, the liberal Warsaw mayor who was standing for Tusk's ruling Civic Coalition (KO), got 49.11 percent, the data showed. Both candidates had declared victory immediately after the publication of an exit poll late on Sunday that showed the result would be very close.

Nawrocki, a conservative historian and amateur boxer who was backed by PiS, had presented the vote as a referendum on Tusk's 18-month-old government.

"The referendum on the dismissal of the Tusk government has been won," PiS lawmaker Jacek Sasin wrote on X.

Poland's blue-chip stock index shed more than 2 percent in early trade on Monday as investors anticipated more political paralysis. The zloty currency also fell versus the euro.

Nawrocki, like his predecessor Duda, is expected to block any attempts by the Tusk government to liberalise abortion or reform the judiciary. The EU took the previous PiS government to court over its judicial reforms, saying they undermined the rule of law and democratic standards.

"Everything was on a knife edge," said 32-year-old IT specialist Patryk Marek. "Feelings are for sure mixed for this moment. But how small this margin was, it tells us how divided we are almost in half as voters."

Sunday's run-off vote in Poland came just two weeks after Romania's centrist Bucharest mayor, Nicusor Dan, had dealt a blow to hard-right and nationalist forces in central Europe by winning that country's presidential contest.

Congratulations poured in from other nationalist and eurosceptic politicians in the region. The defeated hard-right candidate in Romania's election, George Simion, wrote on X "Poland WON", while Hungarian Prime Minister Viktor Orban hailed a "fantastic victory".

The result could lend momentum to the Czech Republic's eurosceptic opposition leader and former Prime Minister Andrej Babis who leads opinion polls ahead of an October election. Babis offered "warm congratulations" on X.

European Commission President Ursula von der Leyen said she was convinced the EU could continue its "very good cooperation" with Poland.

Krzysztof Izdebski, policy director at the Batory Foundation, said the result meant "Trump will have more to say in Polish politics".

Nawrocki, 42, a newcomer to politics who previously ran a national remembrance institute, campaigned on a promise to ensure economic and social policies favour Poles over other nationalities, including refugees from neighbouring Ukraine.

He vowed to protect Poland's sovereignty and railed against what he said was excessive interference in the country's affairs from Brussels.

While Poland's parliament holds most power, the president can veto legislation, and the vote was being watched closely in Ukraine as well as Russia, the United States and the EU.

Borys Budka, a KO Member of the European Parliament, said he believed PiS now sought to "overthrow the legal government".

"This may be a big challenge for the government, which will be blocked when it comes to good initiatives," he told state news channel TVP Info.

Nawrocki won despite his past dominating the last days of the presidential campaign - from questions over his acquisition of a flat from a pensioner to an admission that he took part in orchestrated brawls.

Turnout was 71.31 percent, the electoral commission said, a record for the second round of a presidential election.