Poles backing EU membership have taken part in protests across the country, amid fears it could leave the bloc.
On Thursday Poland’s top court ruled that key articles of EU law were “incompatible” with the constitution.
Organisers said protests took place in more than 100 towns and cities, with some 100,000 people turning out in the capital, Warsaw.
The court’s decision effectively rejected the core principle that EU law has primacy over national legislation.
Poland’s Prime Minister, Mateusz Morawiecki, has defended the ruling. “We have the same rights as other countries. We want these rights to be respected,” he wrote on Facebook after the decision.
He also insisted however that “Poland’s place is and will be in the European family of nations”, and his party has said it has no plans for what some are calling “Polexit”.
Opposition politicians, activists and artists all took part in the demonstrations.
At the rally in the capital, Donald Tusk, former president of the European Council and now leader of the opposition party Civic Platform, called on people to “defend a European Poland”.
Poland stokes fears of leaving EU in ‘Polexit’
Wanda Traczyk-Stawska, a 94-year-old veteran of the Warsaw Uprising against the Nazi German occupation in 1944, spoke at one event.
“This is our Europe and nobody is going to take us out of it,” she said.
Leaders across the EU criticised the ruling by Poland’s Constitutional Tribunal on Thursday.
This decision is the latest in an escalating row between the bloc and Poland.