More than 100,000 Poles demonstrated on Sunday in support of EU membership after a court ruling that parts of EU law are incompatible with the constitution raised concerns that Poland could eventually leave the bloc.
Politicians across Europe voiced dismay at the ruling by Poland's Constitutional Tribunal on Thursday, which they considered to be undercutting the legal pillar on which the 27-nation EU stands.
Organisers said the protests took place in more than 100 towns and cities across Poland and cities abroad, with between 80,000 and 100,000 people gathering in the capital Warsaw, waving Polish and EU flags and shouting, "We are staying".
Donald Tusk, a former head of the European Council and now leader of the main opposition party Civic Platform, said the ruling Law and Justice party's policies were jeopardising Poland's future in Europe.
"We know why they want to leave ... so that they can violate democratic rules with impunity," Mr Tusk said in front of Warsaw's Royal Castle, surrounded by thousands of protesters flanked by police vans with flashing lights.
Populist regimes at odds with liberal EU approach
Right-wing populist governments in Poland and Hungary have found themselves increasingly at odds with the European Commission over issues ranging from LGBTQ rights to migration policy.
"Just as Brexit suddenly became a fact, something no one expected, the same thing can happen here," said Janusz Kuczynski, 59, in Warsaw.
Welcoming the court ruling on Thursday, Polish Prime Minister Mateusz Morawiecki said each member state must be treated with respect and the EU should not be "a grouping of those who are equal and more equal".
The state-run TVP broadcaster, which critics say focuses heavily on presenting the government's point of view, ran a news ticker that read "protest against the Polish constitution" during its coverage of Sunday's events.
Speakers at the demonstrations included politicians from across the opposition, and artists and activists.
"This is our Europe and nobody is going to take us out of it," said Wanda Traczyk-Stawska, 94, a veteran of the 1944 Warsaw uprising against Nazi occupiers.