Large protests were held across Poland on Sunday in a show of support for the European Union after the nation's constitutional court ruled this week that the Polish constitution overrides some EU laws.
Critics of the right-wing nationalist government fear that the court ruling could lead to "Polexit," or Poland being forced to leave the EU over an apparent rejection of the bloc's laws and values.
Thousands of people in Warsaw filled Castle Square in the historic centre, with people chanting, "We are staying!" Some held signs with slogans like "we are Europeans."
Donald Tusk, the top opposition leader in Poland and a former EU leader, called for the protest, casting it as an effort to defend Poland's continued membership in the 27-nation bloc.
He strongly denounced the actions of the ruling party, under leader Jaroslaw Kaczynski, who has been in conflict with the EU for six years as it has sought greater control over the courts.
The EU views the changes as an erosion of democratic checks and balances.
Speaking to crowds, Tusk warned that a "pseudo-court ... by order of the party's leader, in violation of the constitution, decided to take Poland out of the EU."
Kaczynski has denied that he seeks to leave the bloc, though top members of the ruling party have recently used language suggesting that might be their aim.
The state broadcaster, a mouthpiece for the ruling party Law and Justice, ran headlines during Tusk's speech including "Protest against the Polish constitution," and "Tusk attacks Polish sovereignty."
Large protests also took place across the country.
Lech Walesa, who won the Nobel Peace Prize for his opposition to the communist regime, spoke to crowds in Gdansk to applause and chants of "thank you!"
Walesa is a frequent critic of the government, whom he accuses of destroying many of Poland's democratic achievements.
EU membership is extremely popular in Poland, having brought new freedom to travel and a dramatic economic transformation to the central European nation, which had endured decades of communist rule until 1989.
The court ruling, which was issued Thursday by a court largely loyal to the nationalist government, marks a challenge to the primacy of EU law.
The prime minister asked for the review after the European Court of Justice ruled in March that Poland’s new regulations for appointing Supreme Court justices could violate EU law, and ordered the right-wing government to suspend them.