The Ipsos exit poll suggested that the opposition together has likely won 248 seats in the 460-seat lower house of parliament, the Sejm.
Law and Justice, according to the projection, obtained 200 seats, while the far-right Confederation got 12 seats.
Votes were still being counted and the state electoral commission says it expects to have final results by Tuesday morning.
Law and Justice won nearly 44% of the vote in 2019, but has been polling in recent weeks at more than 30%.
In a sign of the emotions generated by the vote, more than 600,000 Poles registered to vote abroad.
Persons:
Donald Tusk, Tusk, Jaroslaw Kaczynski
Organizations:
Justice, Law, Democracy, Poland, ”, Tusk's Civic Coalition, European Union, Votes, EU, Senate, Foreign Ministry
Locations:
WARSAW, Poland, Ukraine, Russia, Polish