JOHANNESBURG, AUG. 13 -- The largest South African mining company, taking a tough new line against striking gold and coal miners, called in riot police today to drive more than 300 workers from a processing plant where they had begun a sit-in.

The giant Anglo American Corp. said the workers were sabotaging the plant and had already caused more than $2 million in damage. Police fired volleys of tear-gas grenades to disperse the strikers and arrested 23 in the resulting melee.

Anglo American also threatened to close as uneconomical a coal mine and one shaft of a major gold mine, with the loss of several thousand jobs, if the 5-day-old strike by the National Union of Mineworkers was not ended by Monday.

The company's own security forces had earlier clashed with strikers at three gold mines, where Anglo American said it was protecting miners who wanted to continue working. Eighteen of the strikers were injured at one of the mines when the security forces fired rubber bullets.

The unexpected moves by Anglo American, which considers itself the most progressive of South Africa's major mining companies, may prompt similar actions by the other companies.

The strike appears to be growing, with at least half of South Africa's 600,000 black miners now participating and more mines and processing plants being affected, according to the independent Labor Monitoring Group.

Cyril Ramaphosa, the mine workers' general secretary, said that about 10,000 more miners had joined the strike today at 52 gold mines, coal pits and processing plants despite what he described as managerial and governmental efforts to break the strike.

The Labor Monitoring Group said it had surveyed 53 gold and coal mines, about half of those in the country, and found that more than 281,000 of the 425,000-man work force were on strike there. Nationwide, the number of miners on strike was well in excess of 300,000, the group said.

The Chamber of Mines, an organization of the six major mining companies, had said that about 230,000 miners were on strike but that some had returned to work today. The union puts the number of strikers at more than 340,000.

The strike, already a historic test of strength, has substantially reduced production of gold and coal, which together earn about two-thirds of the country's foreign exchange.

About 150 union members stopped work today at the Rand Refinery, where gold ore is refined to 99.9 percent purity for export. But a plant spokesman said production continued with its 190 white staff.