Poland must start work on changing its constitution, President Karol Nawrocki said after he was sworn into office.
"Recently, the Polish constitution has been violated so regularly that we, as the political class, must begin working on solutions for a new constitution that will be ready for adoption, I hope and believe, in 2030," he said.
Mr Nawrocki's election victory dealt a blow to Prime Minister Donald Tusk's hopes of cementing the pro-European Union course he has set for the bloc's largest eastern member and left his government floundering in the polls.
Poland is now bracing for a continuation of the deadlock seen under nationalist outgoing President Andrzej Duda, with Mr Nawrocki able to use his veto powers to stymie a government agenda that includes rolling back judicial reforms implemented by the nationalist opposition party Law and Justice (PiS), which critics said undermined the independence of the courts.
Mr Nawrocki also looks set to pose a headache for the government by proposing measures such as tax cuts that are likely to be popular with many voters but hard to implement for an administration with a stretched budget.
"As prime minister, I have so far worked with three presidents," Mr Tusk, who was also prime minister from 2007 to 2014, wrote in a post on X. "What will it be like with the fourth? We'll manage."
The incoming president has said he does not currently see a place for Ukraine in NATO or the EU, a marked shift in tone compared to Mr Duda.
As president, Mr Nawrocki would be required to sign off on Poland's ratification of a new member joining NATO.
While Mr Tusk has said that the European Union should play a bigger role in defence matters alongside NATO, PiS and Mr Nawrocki have argued this would undermine Poland's alliance with the United States.
"The United States is undoubtedly our priority partner," said Mr Nawrocki's spokesman Rafal Leskiewicz.
However, the fact that the former head of the Institute of National Remembrance is a political newcomer who was little known to the public before PiS threw its weight behind him means there is much uncertainty about how his presidency will pan out, political observers say.
"I don't know if he will, in short, fully implement the policies of Law and Justice ... or if he will try to come up with his own initiatives," said Andrzej Rychard, a sociologist from the Polish Academy of Science.
Mr Nawrocki emerged victorious from a tumultuous campaign in which allegations regarding his past, including that he acquired a second property from an elderly man in return for a promise of care that he did not provide, frequently dominated the headlines.
He denied accusations of wrongdoing, although he admitted to taking part in an organised fight between football hooligans, adding to the tough-guy image the amateur boxer had already sought to cultivate.
After the election, supporters of defeated liberal candidate Rafal Trzaskowski filed thousands of protests to the Supreme Court over irregularities at some polling stations. However, the irregularities were not enough to materially alter the result.
PiS accuse their liberal opponents of trying to subvert the will of the people and their supporters gathered in the capital on inauguration day.
"Whoever can - come to Warsaw ... for the swearing-in of the president," PiS MP Michal Wojcik wrote on X. "Let's show that patriots are with Karol Nawrocki on this important day."
