Turnout, at more than 55 per cent, was the highest since Poles voted to end communism in 1989.
Blow for twins
The election was called two years early after Jaroslaw Kaczynski's last coalition collapsed amid acrimony over a corruption investigation.
Lech Kaczynski, the president and Jaroslaw's twin brother, does not face an election until 2010 and has the power to veto legislation.
The Civic Platform's victory over the ruling Law and Justice party appeared to be a rejection of two years of turbulent rule by the nationalist twins in the European Union's biggest former communist country.
The Platform is favoured by financial markets, which expect reforms such as tax cuts and privatisation that would also help the country of 38 million towards adopting the euro currency.
The Platform has said it would seek to pull Polish troops out of the US-led force in Iraq, but does not oppose Washington's plan to put installations for its controversial missile shield in Poland.
Full results are expected on Tuesday. Exit polls tend to be a fair reflection of the eventual result.