Most were Democratic party members who have been camping at the National Assembly since December in an effort to stop conservatives from pushing through measures before the current session ends on January 8.
On December 18, clashes involving sledgehammers and fire extinguishers broke out after the Grand National party, which has 172 seats in the 299-seat assembly compared to the opposition's 83, insisted on the swift approval of a free trade pact, signed by Seoul and Washington last year but awaiting ratification by both legislatures.
The opposition says the pact should not be approved until president Lee Myung-bak's government provides better measures to protect farmers and others who would suffer from US imports.
The GNP says it wants to ease bank ownership rules, privatise state firms and revamp the broadcast industry.
A spokeswoman for parliament's secretariat said it was hoped that the sit-in would be ended and the legislature back to work before Monday.
She added that the secretariat plans to file a criminal complaint if the members do not leave.