Tens of thousands of far-right supporters on Saturday chanted anti-European Union slogans amid a sea of red and white Polish flags at an independence day rally.
The annual event in Warsaw attracted 40,00 people, city hall said, while media outlets estimated 90,000 had turned out, less than a month after the pro-EU opposition won legislative elections.
Banners and placards called for a "Polexit" after Britain's Brexit from the EU from January 2020.
"If there are new changes in the European treaties, Poland must be ready to quit the European Union," said All-Polish Youth leader Marcin Kowalski.
Future changes to the treaties will transform the bloc into a "euro-federalist collective" led by the European Commission, Kowalski charged.
The ruling Law and Justice (PiS) party which had backed the rally in previous years, organised its own independence celebrations this year on the eve of the official date.
The conservative PiS is trying to form a new government after falling short of a parliamentary majority in the October 15 vote, with little hope of finding a coalition partner.
Opposition leader Donald Tusk's liberal Civic Coalition bloc finished second but together with two minor parties -- the centrist Third Way and the Left -- secured a majority of 248 lawmakers in the 460-seat parliament, which is due to re-open on Monday.
Warsaw's relations with the EU's governing institutions have plummeted since the PiS came to power due to perceived back-pedalling on democracy by Warsaw.