SEOUL, SEPT. 9 -- Workers today returned to the assembly lines at the Daewoo and Hyundai motor companies, South Korea's two largest car manufacturers, as labor unrest appeared to diminish throughout South Korea. Operations at both plants had been slowed by internal wage disputes and a shortage of parts caused by strikes at supplier firms.

Government officials said that strikes continued at 269 firms, down from more than 600 last weekend, and most of those were small taxi and bus firms. Today, as on every day this week, more disputes were resolved than broke out.

The trend reversed a summer pattern of spreading labor disputes that officials had said threatened democratization here. Since President Chun Doo Hwan promised on July 1 to allow direct presidential elections and other reforms, more than 3,000 labor disputes have taken place, more than 10 times last year's total, as workers took advantage of the liberalization to seek higher wages and independent unions.

"The trend is downward -- and thank goodness, because people were becoming very worried and apprehensive about it," one diplomat here said. "That's not to say that some of these couldn't start up again, but right now the labor disputes are dwindling."

Opposition leader Kim Dae Jung meanwhile continued his triumphal tour through his home province, attracting huge and peaceful crowds in the southwestern port of Mokpo. Kim then visited the island of Haeui to pay respects at the graves of his parents in his home village.

Meanwhile, ruling party chief Roh Tae Woo, who announced Monday that he would visit the United States next week, backed partly away from his party's claim that he would meet with President Reagan. Party officials said the meeting is not fixed.

A western diplomat here said he believes such a meeting would be a mistake for the United States, which could be perceived as endorsing Roh for president in elections scheduled for December, and for Roh, who could antagonize nationalist sentiment here.

One potentially explosive labor dispute, at the giant Hyundai shipyard in Ulsan, remained unresolved today. Thousands of workers held a sit-in to protest the company's closing of the shipyard and refusal to negotiate further on requested wage increases.

But other companies in the Hyundai conglomerate resolved their disputes, defusing tensions in the east coast industrial city. Hyundai, which produces autos for export to North America, resumed operations after a 15-day hiatus, its second of the summer. Company officials said a shortage of parts this summer caused by labor strife had cut average daily production from 2,200 vehicles to 900.

The lessening of labor unrest followed a government crackdown against union leaders accused of inciting violence and students who have taken factory jobs to help organize unions.

The government today for the first time used the tough National Security Law in a labor dispute, charging two students-turned-workers with instigating labor violence. The charges brought the number of recently arrested workers to 372.

One diplomat said the arrests, primarily at Hyundai and Daewoo, which is near Inchon, may have provided a useful cooling-off period.