According to Warsaw city hall, 50,000 people took part in the march but police said that estimate was exaggerated.
People marched holding European Union and rainbow flags and chanting “freedom, equality, democracy” and “together, with fists raised, we will take down the dictatorship”.
The Leader of the opposition Civic Platform party, Grzegorz Schetyna, said people had gathered in Warsaw to give Poles hope that “the end of [the conservative] Law and Justice (PiS) is near”.
“We will fight for freedom, for dignity, for democracy and for Poland in the European Union,” Schetyna said, adding: “We fight for the most important issues: the constitution, the independence of the Constitutional Tribunal, the independence of courts, democracy, free elections”.
PiS introduced a swathe of changes to Poland's judiciary since it came to power in late 2015.
Katarzyna Lubnauer, head of the Nowoczesna party, which organised the march together with Civic Platform, said she was certain that the opposition would defeat Law and Justice in the next four elections.
“Check-mate in four steps – first the local government elections, then to the European Parliament, then parliamentary elections [in Poland] and then Polish presidential elections. PiS will lose and we will win,” Lubnauer said.
Participants in the march also voiced support for an ongoing protest for greater benefits for people with disabilities.
Since April 18, a number of people – mainly the parents of disabled adults who are unable to care for themselves – have been occupying a hallway in the parliament building. They are demanding a monthly PLN 500 (EUR 120) handout to cover ongoing therapy costs. (vb)