The government says it will begin enforcing laws next year to allow multiple unions for each workplace and prohibit companies from paying wages to full-time union representatives. The laws' implementation has been delayed for more than a decade because of opposition from labor organizations.
The protesters chanted slogans against the government's move during the rally near the legislature. Some shaved their heads in a show of their determination.
"We will thwart the government's policy by mobilizing every means possible. If necessary, we will stage a general strike," Kang Choong-ho, a spokesman for the Federation of Korean Trade Unions, said after the rally.
The 900,000-member FKTU is one of South Korea's two major umbrella labor organizations.
Kang also accused the conservative government of President Lee Myung-bak of trying to weaken labor unions.
Repeated calls to the Labor Ministry seeking comment went unanswered Saturday.
Kang said some 150,000 protesters participated in the rally while police estimated the crowd at 60,000 people.
No clashes with police occurred and the protesters later peacefully dispersed.
