Seoul, South Korea ? Fire bombs lit up the evening sky in Seoul on Sunday as labor advocates and students battled riot police in one of the most violent protests in years. Dozens of students and workers were injured, witnesses said.

Police hauled away dozens of workers and students bleeding from their heads, while protesters lobbed hundreds of fire bombs.

The Korean Confederation of Trade Unions, which organized Sunday’s protest, said at least 43 workers were hospitalized, one of them unconscious. Police reported 16 officers injured, said South Korean news agency Yonhap.

In one clash, hundreds of police cornered a score of students in an alley and pummeled them with plastic shields and batons. Television footage showed police stomping on protesters on the pavement.

Hundreds of students and workers regrouped in an eight-lane boulevard and its side alleys, chanting: “(President) Roh Moo-hyun, stop oppressing workers!”

Sunday’s protest signaled a resurgence of labor unrest the government fears would drive away foreign investors. Fire bombs, once a common tool of protest in South Korea, have been absent from Seoul streets in the past 1 1/2 years.

A crowd of protesters, estimated by police at 35,000 and by the labor confederation at 100,000, rallied in central Seoul earlier Sunday to protest damages lawsuits that managers have filed against union leaders accused of staging illegal strikes.