Some 30 million voters are eligible to determine the shape of the bicameral legislature, although there have been predictions of a low turnout. There are 460 seats up for grabs in the Sejm, the lower house of parliament, and 100 Senate seats.
No party is expected to score an outright majority, but opinion polls suggest the center-right Civic Platform may fare best. Led by Donald Tusk, the opposition party plans to boost entrepreneurship, pull troops out of Iraq, and rebuild relations with EU allies that have suffered under the ruling party.
The conservative Law and Justice party's two years of rule have been marked by growing prosperity, but also constant political turbulence.
Twin Peaks, Valleys
Led by Prime Minister Jaroslaw Kaczynski and his twin brother, President Lech Kaczynski, the party has campaigned for Poland to host part of a planned U.S. missile-defense shield, further harming already strained relations with Russia.
No party is expected to score an outright majority, but opinion polls suggest the center-right Civic Platform may fare best.
The Kaczynski government, which took power just a year after Poland joined the European Union, has also openly clashed with older members of the bloc.
The last coalition government collapsed in September, prompting the prime minister to push for a parliamentary election two years early. The president does not face an election until 2010.
The opposition Civic Platform can count on greater support in cities and among the young. The power base of the socially conservative Kaczynskis is among older Poles, devout Catholics, and people in rural areas who feel left out by the changes since the end of communism.
But many people have been disappointed with the two-year rule of the twins and are looking for a change.
"Those ducks [a nickname for the Kaczynski twins] don't do anything at all," says Marek Kacperovski, a construction worker in Lowicz, a rural town west of the capital. "Right now we should give a chance to something new. Law and Justice during the last two years did not do anything. I hope the Civic Platform will do something."
If opposition parties win three-fifths of the lower house's seats, they will be able to nullify the president's power to veto legislation.
Voter Apathy Eyed
Unofficial exit polls will be made public after polling stations close this evening. Official results are expected to be announced by October 23.
Turnout could be crucial. Only 40 percent voted in the 2005 election, a showing that was seen as having boosted Law and Justice.