Thousands of people across Poland demonstrated against a controversial ruling by the Constitutional Court and for their country to remain in the European Union.
In Warsaw, demonstrators gathered in the Castle Square. They waved Polish and EU flags, shouting "We stay" and "We are Europe!"
There were also protests in Danzig, Poznan, Szczecin, Krakow and many other cities.
The country's Constitutional Court recently ruled that certain elements of EU law violate the Polish Constitution. It thus gives national law precedence over EU law.
This decision further fuels the conflict between the European Commission and Warsaw over the reform of the Polish judicial system.
Former European Council president and Polish opposition leader Donald Tusk called for the protests.
During his appearance in front of the demonstrators, he accused the conservative ruling party PiS of wanting to lead the country out of the EU.
"Poland's place is in Europe," Tusk told protesters in Warsaw, "We will win because we are the majority."
Both Tusk and other speakers were repeatedly interrupted by loud heckling and chants from a counter-demonstration of far-right nationalist groups.
In Danzig, Nobel Peace Prize laureate and former Polish President Lech Walesa addressed the demonstrators.
"The people who run the state today are a great misfortune for Poland," Walesa said. No enemy that has ever ruled Poland has divided the people of the country like the PiS, he added.
The PiS government has been restructuring the judiciary for years. Critics accuse the party of putting judges under pressure.
The European Commission has already opened several infringement proceedings against Warsaw because of the reforms and has filed lawsuits at the European Court of Justice.