Poland's ruling Law and Justice party (PiS) is predicted to lose its majority to the Civic Coalition lead by former European Council president Donald Tusk.
Seventy-four percent of Poles cast their ballots, the highest level in the country's 34 years of democracy, and surpassing the 63% who voted in the historic 1989 election that toppled communism.
According to exit polls, Prime Minister Mateusz Morawiecki's PiS party has won more than 36% of votes, but is unlikely to find enough coalition allies to retain office.
The opposition Civic Coalition won 31%; the centrist Third Way coalition 14%; the Left party 8.6%; and the far-right Confederation about 6%.
The Ipsos exit poll shows that Poland's three biggest opposition parties have likely won a combined 248 seats in the 460-seat lower house of parliament, the Sejm.
The poll suggests that Law and Justice obtained 200 seats. Its potential partner, the far-right Confederation got 12 seats, a showing the party acknowledged was a defeat.
Tusk, who governed as Polish Prime Minister from 2007 to 2014, has already claimed victory.
The final election results should be known by Tuesday.
''Never in my life have I been so happy about taking seemingly second place. Poland won. Democracy has won. We have removed them from power,” Tusk said.
He hopes to end eight years of Law and Justice party rule under President Jaroslaw Kaczynski.
A political change in Poland could open the way for the release of billions of euros of Covid-19 pandemic recovery funds that have been withheld over what the EU viewed as democratic erosion.
Should the polls be correct, Tusk’s victory would move Poland closer to European allies and bolster support for Ukraine's defence against Russia.