The Law and Justice (PiS) party won 43.6 percent of the vote, according to the survey by pollster Ipsos.
Parties from across the political spectrum have said the election is the most important since the collapse of communism in 1989 and that it would decide the future direction of country riven by bitter political divisions between conservatives and liberals.
The official election results were expected to be announced by Tuesday.
(pk/gs)
More to follow