Striking workers halted production at three of Chile's four main copper mines today, and President Augusto Pinochet responded by promising concessions to moderate critics but harsh actions against opposition leaders.

Earlier, the government filed court charges against 11 leaders of the Confederation of Copper Workers in an effort to end the strike--unprecedented in the decade of his rule--and four days of militant political protest.

Pinochet, in a television broadcast tonight, said measures would be adopted to "severely and exemplarily punish" the leaders of the protest "so that these events do not happen again."

At the same time, he moved to regain slipping support from conservative groups by announcing several measures of political liberalization. Pinochet said procedures governing exiles would be revised to allow some of the thousands of Chileans abroad to return, and he added that some governmental meetings would be publicized and prior censorship on book publishing would end.

The 67-year-old leader did not specify the actions he has been threatening for several days against opposition leaders, saying they would be announced Saturday by the Interior Ministry. Instead, Pinochet reiterated that he intended to enforce his government's ban on political activities and declared he would not consider altering his plan to continue his rule until 1989.

Pinochet's speech appeared designed to defuse the protest movement by isolating the leaders of the opposition while offering conciliation to moderate and conservative followers. Leaders of the Chilean political right have been calling on the government to liberalize its authoritarian rule for several months.

Government officials today reiterated that thousands of striking employes in state-owned mines would be fired immediately. Labor leaders said almost 1,000 miners received dismissal notices yesterday in the first day of a strike at the El Salvador mine complex, about 700 miles north of Santiago.

Copper workers threatened to extend today's 24-hour strike indefinitely. Students at four campuses of the University of Chile announced strikes and held demonstrations today. And several political leaders accused the government of having fomented the violence at what began as peaceful protests Tuesday.

Three days after the massive violent protests shook major cities, the focus of government opposition shifted to the strikes at the three state-owned copper mining centers that together employ about 23,000 unionized workers and produce more than 1 million metric tons of copper annually.

While government and union reports on the extent of the copper strike differed widely, reporters and other observers said that production was paralyzed at each of the three struck centers and that large majorities of blue-collar unions respected the walkout.

The Confederation of Copper Workers, all but one of whose leaders remained free this evening pending court hearings on government charges, said between 97 and 100 percent of its workers stayed off the job. It added that security, logistical and health employes worked with the union's consent and that higher percentages of white-collar administrative employes reported.

The government's copper corporation, Codelco, reported "normal" activities in all three mines and said absenteeism ranged between 15 and 33 percent--the latter at the smallest main mine, Andina, not far from the capital. There were no reports of violence by tonight. The government mobilized troops at some sites and trucks mounted with machine guns guarded buses carrying workers to their jobs.

Copper accounts for more than 40 percent of Chile's export earnings, and an economic analyst said production losses amounted to several million dollars a day.

At El Teniente mine, near the snow line of the Andes Mountains 35 miles south of Santiago, all work halted in the mining and production areas. As the afternoon shift reported, only about two dozen workers boarded the 17 buses that normally are full of miners.

Union leaders, who had worried that many rank-and-file workers might not accept the strike because of fear of losing their jobs, appeared jubilant. "Despite all the threats we have received, and even if we didn't believe up until yesterday we would be successful, we will win this time," said Robemil Aranca Florel, an official of one of the five unions striking at the mine.

Unions at El Salvador mine announced extension of their strike indefinitely to protest the firings of those who refused to work yesterday. It was not clear, however, whether the copper workers' and other labor leaders organized into a National Workers' Command would succeed in mounting a national strike against the government.

Workers at Chile's largest copper mine, Chuquicamata, did not join today's strike and were still deliberating tonight whether to act next week. Other copper union leaders and directors of unions in such key sectors as transport and agriculture were expected to keep workers on the job over the weekend before settling on action on Monday.

Rodolfo Seguel, 29, a copper union leader who was transformed into a symbol of opposition after leading two national days of protest last month and this week, sent a message to workers today from a jail saying, "Our cause is just, let us not falter. Let us not defraud Chile."

It was Seguel's detention, ordered by federal judge Wednesday, that triggered the strike by copper workers. They had vowed in April to act if any member was jailed. The copper workers and other opposition union federations backed away from a national strike last month, however, and many unions have been thought by analysts to be unprepared for full-scale strikes.

Some opposition political parties and other groups have been slow to back the strike attempt, and some opposition activists said today that they believe the escalating confrontation with the government could lead to a sharp reverse for the nascent and still fragile opposition movement.