Some 200,000 unionized workers are expected to join a nationwide strike today organized by the Korean Confederation of Trade Unions (KCTU), the nation's second-largest umbrella union.
The union said the participants will include some 10,000 members of the Korean Teachers and Educational Workers' Union.
The KCTU plans to hold massive rallies in major cities, including Seoul, for the next few weeks.
The government vowed to deal sternly with the striking workers, saying the walkouts were "illegal."
"We are ready to crack down on those who are joining the illegal strike," the government said in a statement, Thursday. "Especially, there will be no leniency for unionized teachers and public workers who are taking to the street."
The KCTU said the strike is in protest against what it called the government's "business-friendly" approach in handling key labor issues.
In a vote in which some 428,800 members cast ballots from March 21 to April 10, 361,743 workers (84.35 percent) voted in favor of the strike.
"The strike will continue for an indefinite period," a KCTU spokesman said.
Meanwhile, the union of Seoul National University Hospital also went on strike Thursday, opposing the hospital's recent move to introduce a performance-based salary system.
Some 400 of the 1,600 union members participated in the strike.
The union said a minimum number of staff will work in emergency rooms and intensive care units despite the strike.
"We have no other option but a strike as the hospital has not fully engaged in the arbitration of the labor commission as well as negotiations with the union," the union said in a statement.
The hospital reported to the labor ministry in February that it would change its wage structure into a performance-based system. It also plans to limit the promotion of underperforming workers.
The hospital said that the strike will not cause serious problems in operating the hospital because few doctors and nurses have joined the strike.