"We will thwart the government's policy by mobilising every means possible. If necessary, we will stage a general strike," Kang said.
Kang also accused the government of Lee Myung-bak, the president, of trying to weaken labour unions.
Riot police were deployed, but there were no clashes and the protesters later peacefully dispersed.
The labour laws' implementation has been delayed for more than a decade because of opposition from labour organisations.
South Korea's two biggest trade unions have joined forces to protest against the laws, which opponents say will undermine workers' rights.