Huge voter-turnout is predicted in Thursday's emotionally charged election being held weeks after the impeachment of President Roh Moo-hyun by parliament.
The controversial impeachment has overshadowed debate over South Korea's participation in Iraq as a partner to the US-led occupation.
"I voted for the Uri Party," said Park Sae-ran, a female banker at a polling station in the east of the capital Seoul.
"We have long seen how corrupt and incompetent old politicians have been. If someone did wrong, he or she should be held accountable," she said. "This election is a change to show that power comes with responsibility."
Foreign investors are watching the election because they fear instability in a major US ally and the world's twelfth largest economy.
The Uri Party is widely expected to win a majority in the 299-seat unicameral parliament, in part through a sympathy vote for Roh, who has played no part in the election.
But even Uri Party officials say their contest with the conservative Grand National Party is too close to call.
"I am a supporter of the Grand National Party because I want stable politics, said Cho Kil-soo, a taxi driver. "The country needs balanced opinions from the old and young."
Analysts say the turnout would be crucial. A higher turnout could mean more young people, many of whom are pro-Uri.
Parliament impeached Roh on 12 March for violating an election law by speaking in favour of the Uri Party. The Constitutional Court has yet to rule whether to uphold the vote.