Crowds of people have been pictured waving Polish and EU flags and holding placards, and participants told the BBC that protesters were chanting “democracy” and “we will win”.
“I came here to defend democracy because I can’t stand how our parliament, the constitutional tribunal are destroyed, the European Union is diminished,” one protester told Reuters news agency.
Donald Tusk, a former head of the European Council, also welcomed supporters during the “record” march.
“Democracy dies in silence but you’ve raised your voice for democracy today, silence is over, we will shout,” said Mr Tusk.
Ahead of the event, the PiS accused organisers of of hypocrisy, external, tweeting a video about police brutality and violence against the media while Mr Tusk was in office.
Wojciech Przybylski, editor of Visegrad Insight, told the BBC that these protests show Poland’s opposition groups can unite over common causes, despite their political differences.
But PiS is still ahead in opinion polls, he adds, and “this is going to mobilise them, because they know the opposition is for real”.