The rally took place in the Polish capital on Thursday afternoon, the state news agency PAP reported.
Jarosław Kaczyński, the leader of the Law and Justice party which ruled Poland between 2015 and 2023, said the rally was being held "in defence of freedom of speech, media and democracy."
He accused the new pro-European government of Donald Tusk of “breaking the rule of law, the constitution, trampling the law,” adding that his party must win the battle for a "sovereign and independent Poland," according to news outlets.
The rally also called for the release of two Law and Justice MPs, the former interior minister, Mariusz Kamiński, and his deputy Maciej Wąsik, who were imprisoned on Tuesday to serve a two year sentence for abuse of power during Law and Justice's previous spell in office in 2007.
Earlier in the day, President Andrzej Duda, a Law and Justice ally, said he would pardon the two ex-ministers.
Law and Justice said Kamiński and Wąsik were "political prisoners," while the government maintains they had been properly convicted and the only way for them to avoid prison, is for the president to pardon them, the Politico news service reported.
Senior Law and Justice politicians told reporters that Thursday's protest outside parliament had been attended by over 200,000 people.
Warsaw City Hall put the figure at 35,000, the PAP news agency reported.
Source: PAP, Business Insider, Politico