Thousands of riot police surrounded two university campuses where union activists taking part in a strike by state rail and power workers were holed up on Tuesday.
Calling the strike illegal, authorities said they planned to arrest 36 leaders of the walkout who were taking refuge at a Roman Catholic church in central Seoul.
Thousands of auto and machinery workers prepared to walk out later Tuesday to support the rail and electricity workers, who went on strike Monday to demand a shorter work week and oppose a government privatization plan.
There were heavy disruptions in rail traffic Tuesday, but the impact of protests by power workers was limited because their facilities are mostly automated.
Privatization of ineffective state firms is a priority in President Kim Dae-jung's economic reform programs. Kim started the reforms in the aftermath of the 1997-98 Asian economic crisis. Workers fear the changes will lead to mass layoffs.
Many striking workers, some armed with iron bars, gathered at Seoul National University and Konkuk University. Colleges are traditional rallying points for anti-government activists, a legacy of South Korea's pro-democracy student movement and the reluctance of police to enter the campuses.
Gas workers pulled out of the strike hours after it began Monday after reaching a compromise with management.
Union leaders said they expected up to 100,000 workers to join the strike on Tuesday.
"The government should listen to the desperate voice of workers demanding job security," said Lee Nam-soon, head of the Federation of Korean Trade Unions, South Korea's largest labor group.
Lee said he met Labor Minister Bang Yong-suk in overnight negotiations and was to meet him again Tuesday morning before deciding whether to organize a sympathy strike later in the day.
The walkout by rail workers forced authorities to cancel 50 percent of all passenger and freight train operations for the second straight day, officials said.
Nearly a quarter of the 20,000 unionized rail workers were taking part in the strike. They included 1,500 locomotive operators, about half of the total.
Rail commuters in the Seoul metropolitan area faced delays. Major roads around large cities were jammed with motorists who could not use trains.
Shipment of export and import goods was delayed because of the rail strike. Trucks were used to transport shipments to and from seaports.
The government has proposed introducing a five-day work week. But labor and management differ over whether to cut wages and paid vacations in return for the shorter work week.
Most South Koreans work a half day on Saturday.