JOHANNESBURG, AUG. 10 -- Striking black miners shut down production at more than a third of South Africa's gold and coal mines today to support their demands for higher wages and safer working conditions.

The National Union of Mineworkers said that 340,000 of the country's 600,000 black miners, including many who are not union members, had stopped work. Support for the strike was growing and could spread soon to mines where the union is not recognized, the union said.

The South African Chamber of Mines, which had predicted earlier that relatively few miners would heed the strike call, acknowledged tonight that at least 40 percent of the country's black miners were on strike and that the largest mines were the hardest hit.

The strike, the largest ever in South Africa's vital mining industry, could cripple the country's economy if prolonged. Together, gold and coal exports account for more than half of its foreign exchange earnings, and even in a recession, mining is the strongest element of the economy.

The strike has strong political overtones, since the miners view their demand for a "living wage" as part of the broader struggle against the apartheid system of racial separation and minority white rule.

{The South African Parliament voted to repeal legislation that prevents blacks from rising above a certain job level, thus keeping them underground while whites monopolize supervisory and technical jobs, Agence France-Presse reported from Cape Town. Among the discriminatory practices to be ended -- provided President Pieter Botha signs the repeal bill -- is a regulation barring blacks from obtaining blasting certificates. These are a prerequisite to advancement.}

Work effectively was halted at 44 of the 46 struck mines, the union said, noting that these mines produce more than half of South Africa's gold and 20 percent of its coal. The Chamber of Mines maintained that only 31 of its members' 99 mines had been closed by the strike, but agreed that a prolonged strike at these would have a substantial economic impact.

"The strike is totally successful," said Cyril Ramaphosa, the union general secretary. "It is a very big step toward a major victory."

Scattered violence was reported at several gold mines east and west of Johannesburg as strikers clashed with miners who stayed on the job and with mine security forces attempting to protect them. Union officials said workers at two mines had been forced underground at gunpoint, but company spokesmen denied this.

About 15 persons were injured in a dozen incidents, according to the company spokesmen, and as many as 20 union officials and miners were arrested. Union officials, complaining of harassment, acknowledged the arrests and accused the police of interfering with the strike and assisting management.

But police headquarters in Pretoria declared tonight that "no action was taken on any mine anywhere in the country."

"The Chamber of Mines has consistently tried to crush the strike," Ramaphosa told a news conference, "but the campaign for a living wage has now started in earnest."

The miners are seeking a 30 percent increase in wages, danger pay, improved working conditions and longer vacations. The six major mining houses have offered -- and unilaterally provided -- pay increases of 17 to 23 percent, bringing the average black miner's wages to about $253 a month. This is about one-third of the minimum pay of a white miner.

Negotiations broke down last month, with each side accusing the other of refusing to bargain in good faith.

"We have always said that we would negotiate, but there is a point where negotiations become non-negotiations," Ramaphosa said. He criticized the mining companies' practice of terming a proposal a "final offer," putting the wage increase into effect unilaterally and then declaring an end to the negotiations. "Mineworkers are determined to show . . . they are prepared to mount a bitter and protracted struggle for a living wage," he said.