SEOUL, AUG. 17 -- The giant Hyundai combine, South Korea's No. 1 auto maker, shut down six industrial plants today, touching off labor violence in the southern city of Ulsan.

An estimated 20,000 workers broke into the compound of the Hyundai shipbuilding subsidiary, the nation's largest, smashing through the guardhouse beside the main gate. The gate had been blocked by company officials, who used a crane to place a 1,000-ton section of a ship's hull in front of it.

Later, in heavy rain, about 1,000 workers left the yard and, hurling rocks and bottles, tried to march on the center of Ulsan, according to press reports. They were turned back by riot police firing tear gas.

The step taken by Hyundai, which shut down the conglomerate's major operations for an indefinite period, marked a new level of conflict in the labor strife that has crippled South Korean industry in the past few weeks. For the first time, organizers were pushing for recognition of a labor union representing all workers of an industrial group.

Hyundai officials charged that the prospective union was formed illegally, and insisted that pro-company unions were already established at all Hyundai subsidiaries. The specific reason given for Monday's action, however, was to protect company property and what officials said was a majority of workers who opposed the new union, which was formed late last week.

The plants shut down today produce heavy machinery, ships, precision tools and electrical and wood products.

Hyundai Motor Co., producer of the Excel subcompact car that is exported to the United States, was closed last week. Officials said production could not continue because labor disputes had disrupted supplies for its assembly line. Daewoo Motor Co., the country's No. 2 auto maker, in which General Motors owns a 50 percent share, also shut down last week and failed to reopen today because of parts shortages.

The developments at Hyundai took place as the nationwide labor strife showed signs of easing. The Labor Ministry said the number of disputes resolved over the weekend exceeded new walkouts for the first time in three weeks. However, the number of work sites still affected by labor disputes remains high. The ministry put the figure at 280.

The strikes began in early June, in tandem with antigovernment demonstrations calling for political reform. When the government party agreed in late June to meet the major opposition demands, the walkouts accelerated.

Many business leaders say that unionists are former student radicals who left university campuses to infiltrate the labor movement. Other analysts said the walkouts were sparked by the promise of political change.

Nearly all politicians and analysts, including those of the ruling party -- which supported management with tough antistrike regulations in the past -- agree that the blue-collar worker deserves a greater share of South Korea's growing economy.

The central Bank of Korea reported today that the economy's growth rate increased by 15 percent in the second quarter.