Hundreds of thousands of people marched in Warsaw under the banner "we are and will remain in Europe", even as scores of nationalists held a protest against the European Union.
More than 1,000 Polish nationalists and followers of right-wing Catholic groups gathered in Warsaw at midday Saturday to demonstrate against perceived EU interference in national politics.
Counter rallies by backers of the 28-member bloc attracted around 240,000 people, according to the city administration, in what has been the largest public demonstration since Poland's national-conservative government came to power.
"We are here because we believe in Polish law, Polish freedom and common action in the EU," former Polish president Bronislaw Komorowski said at the beginning of the rally, which had support from both left-wing and conservative opposition parties.
The policies of the ruling Law and Justice party (PiS) have strained relations between Warsaw and Brussels in recent months as the EU investigates controversial new laws on public broadcasters, the constitution and police powers.
"We are here together to say that we will not succumb to the nightmare of an authoritarian government," former foreign minister and head of the liberal Civic Platform (PO) Grzegorz Schetyna told the pro-EU crowds.
"The government of PiS is turning away from Europe," Kamila Gasiuk-Pihowicz of the liberal-conservative party Nowoczesna said.
Protesters from the other camp came out in support of the government and marched against EU interference in sovereign matters.
Warsaw-based medical student Kondrian, 24, wants Poland to regain its full sovereignty. "It would be best if we left the EU," he told dpa ahead of a mass in St Florian's Cathedral, where anti-EU protesters were gathering.
"We are not against Europe, but we don't want anyone telling us what to do," said 50-year-old engineer Piotr Goszczynski, adding this was true both for the acceptance of refugees as well as national politics.