EU Council President Donald Tusk joined the opposition and thousands of people to encourage a vote in favor of the EU. The ruling Law and Justice party has brought division over media, migration and justice policies.
Thousands of people turned up in the Polish capital, Warsaw, on Saturday to show support for the European Union and encourage people to vote in Sunday's European Parliament election.
Speaking at the rally, EU Council President Donald Tusk encouraged people to vote for pro-EU parties, saying it would be a vote for a "free, democratic, Poland without fear, without constraint, without contempt for others."
It was the second time within weeks that Tusk had appeared to criticize the ruling Law and Justice (PiS) party for disregarding the country's constitution and to stand up for the EU. "A secure Poland in a strong and united Europe is the most precious political value for Poles today," Tusk said.
The Polish government has been in power since 2015 and found itself in a dispute with the EU over changes made to the judicial system that it claims are needed to remove influences left over from the Soviet-era. Critics claim it is a way to bring the courts under closer political control.
Both the EU and the Council of Europe human rights group have warned that some of the government measures violate Poland's constitution.
Presidential candidates 2020
Tusk, a former Polish prime minister, ends his term as EU Council president in December and opinion polls indicate he and current President Andrzej Duda are in a tie ahead of the 2020 Polish presidential elections.
Poland has been one of the EU's major net beneficiaries and about 85% of voters back its membership of the bloc. But turnout for the European elections has been well under 30% in every vote since Poland became a member in May 2004.
jm/sms (AP, dpa)