Poles who support EU membership have protested around the country amid fears that the country could quit the bloc.
Poland's highest court declared on Thursday that crucial EU laws were "incompatible" with the country's constitution.
It fueled fears that Poland may leave the EU, sparking demands for widespread protests on Sunday.
According to organizers, protests were held in over 100 towns and cities, with over 100,000 people in Warsaw alone.
At a demonstration in the capital, Donald Tusk, the former President of the European Council and now the head of the opposition Civic Platform, urged Poles to "defend a European Poland."
The demonstrations were attended by opposition politicians, activists, and artists.
Reuters At one of the events, Wanda Traczyk-Stawska, a 94-year-old veteran of the 1944 Warsaw Uprising against Nazi German occupation, spoke.
"This is our Europe and nobody is going to take us out of it," she declared.
Leaders from around the EU slammed Poland's Constitutional Tribunal's decision on Thursday. In effect, it rejected the fundamental idea that EU law takes precedence over national law in some regions of the law.
This is the latest in a series of increasing spats between the EU and Poland.
Since gaining office in 2015, critics have accused the ruling Law and Justice (PiS) party of politicizing the judiciary.
The Constitutional Tribunal in Poland is presently dominated by judges who support the ruling party. One of them was allegedly appointed illegally, according to the European Court of Human Rights.
Prime Minister Mateusz Morawiecki, on the other hand, has defended the judge's contentious decision.
"We are entitled to the same rights as other nations. We demand that these rights be upheld "Following the judgment, he posted on Facebook.
However, he stressed that "Poland's place is and will be in the European family of nations," and PiS has stated that it has no ambitions for "Polexit."