Tens of thousands have marched in central Warsaw in protest at the government's policies, which they view as anti-democratic and harmful to Poland's position in the EU.
City authorities estimated that 240,000 turned out, which if true would make it the largest anti-government demonstration since the Law and Justice party took power last year and embarked on making deep changes to the country.
There was also a counter-protest held by nationalists who voiced opposition to influence by Brussels in Poland's affairs, but it drew significantly fewer people.
The protest and counter-protest highlight a bitter social divide in Poland between those who want deeper integration with Europe and those who feel that EU membership has eroded national sovereignty, only recently regained after the end of the Cold War.
The anti-government march was held under the motto "We are and will remain in Europe" and comes just days after the 12th anniversary of Poland joining the EU.
It was organised by the opposition party Civic Platform and the civic group Committee for the Defence of Democracy, known in Poland as KOD, and was supported by other opposition parties.
KOD was created in November, shortly after Law and Justice took power and began taking steps that have weakened the Constitutional Tribunal.
That move and others which have helped the party centralise power have been condemned by the EU and the Council of Europe, a human rights group, which says the government in Warsaw is eroding the rule of law.
The party says its changes are aimed at building a stronger nation free of what it says is the continued influence of former communists and liberals. Its leaders argue that it has a mandate for change after decisively winning elections last year.
KOD has since organised a number of anti-government marches, and Saturday's large turnout indicated that the segment of Polish society opposed to the changes remains determined.
However, Law and Justice also remains very popular with its conservative electorate. Its supporters object to the anti-government protests that have been held over the past months, accusing organisers of trying to destabilise Poland.