Copper workers walked off the job at a major Chilean mine today, and as union leaders struggled to organize a national strike, President Augusto Pinochet promised to apply the "strong hand of the government" to the opposition movement challenging his rule.

Two days after the largest and most violent antigovernment protest in a decade of military rule, Pinochet said that 1,000 copper workers striking today and the thousands planning to walk out Friday would be fired. He also promised a sharp crackdown on labor leaders and politicians.

"Surely . . . we are going to apply what all of you are asking for, the strong hand of the government," Pinochet told a crowd in the town of Vallenar. "Behind all this are the politicians . . . . We are going to take a few measures, and I think it will be bad for them because it can't be accepted that they go on acting . . . . This has to end once and for all."

Pinochet's angry comments came as workers struck at the mining center of El Salvador, about 700 miles north of Santiago. Union leaders said they planned to go ahead with 24-hour strikes at two of the three other major mining centers Friday. The copper workers' strike was triggered by the arrest yesterday of Rodolfo Seguel, the copper workers' union chief and the leader of the union coalition that organized Tuesday's protest.

Labor leaders are seeking to escalate what has become a major political confrontation with Pinochet by organizing a national general strike. The copper union had hoped that four other labor confederations in the coalition would join the strike Friday. But only one other union group has announced that it will participate, and union leaders from Chile's largest copper mine, Chuquicamata, said they would postpone any walkouts until at least next week.

It was not immediately clear what effect the strike call was having in El Salvador. But union and political sources said the labor coalition had been pushed toward a strike it had hoped to avoid and would have difficulty sustaining its action against strong government repression.

The acting president of the Confederation of Copper Workers, Hugo Estivales, nevertheless said that the firing of striking workers "will only lead us to larger actions." Other sources said the labor coalition, called the National Command of Workers for the Protest, was contacting other key Chilean unions in an effort to organize a general strike for next week.

At stake in the confrontation is the future of a mass political opposition movement led by labor that grew from the national protest Tuesday and another one last month.

In Tuesday's demonstration, the labor coalition, which nominally represents about 70 percent of organized Chilean workers, was joined by political parties, professional groups and thousands of average citizens in day-long demonstrations demanding economic and social reforms and a quick return to democratic government. Pinochet is currently scheduled to rule until at least 1989.

Chilean police announced today that four persons were killed by "unknown elements" in Tuesday night's protest, which brought thousands to the streets to build barricades, set fires and battle police. The government says 1,350 persons were arrested around the country and 19 policemen were injured in Santiago. Of the 634 arrested in Santiago, 26 were being processed by military courts for assaulting police and 234 were being prosecuted for violating Chilean internal security laws, authorities said.

Chile's Catholic Church-supported human rights organization, the Vicariate of Solidarity, said six persons remain hospitalized today with gunshot wounds, and officials said they had received reports from witnesses of Chilean police opening fire into protesting crowds Tuesday night.

Tonight, Chilean news media reported that at least 12 persons had been arrested in renewed demonstrations following the funeral of a 14-year-old boy killed Tuesday night. The reports said police fired tear gas to disperse the crowd that gathered after the funeral.

It was not clear this evening what specific action Pinochet plans to dismantle the protest movement in addition to firing the strikers. Seguel was ordered held by a judge on charges of exciting actions against national security.

In Washington, State Department spokesman John Hughes said Seguel has been allowed to see his attorneys and family in jail. He called for the Chilean government to respect Seguel's political and civil rights in proceeding with the case.

In recent times, the government frequently has expelled opposition politicans and labor activists from Chile and sent many other militants into internal exile in remote areas.

Government officials and leaders of the copper workers' union sharply disputed the success of today's strike in the El Salvador mines, which was called by local unions a day in advance of the national walkout following the dismissal there Wednesday of 23 workers.

Gen. Capricio Torrez, the executive vice president of Codelco, Chile's state copper corporation, told reporters this morning that all but 550 of the 3,400 workers scheduled to report this morning had arrived. He said the mines continued to operate.

In contrast, union leaders said the El Salvador mines had been paralyzed. In a telephone interview, a union leader in El Salvador said that 115 of 880 workers had reported to the principal mine operation this morning and that refining and processing operations were 97 percent closed.

According to reports tonight, about 1,000 workers at the El Salvador mine received dismissal notices.