JOHANNESBURG, AUG. 12 -- South African police today arrested at least 78 black trade unionists on charges they planned to murder gold and coal miners who refused to participate in a nationwide wage strike.

The National Union of Mineworkers, which denied the allegations, put the number of those taken into custody at 86.

Earlier, the union reported the wage dispute -- the biggest in South Africa's history -- had spread to a few more small pits and more than 340,000 men were on strike. The mine owners put the figure at about 230,000.

A statement by police headquarters in Pretoria said the union members were arrested during a raid on union offices at Klerksdorp, west of Johannesburg.

Police said those detained would be charged with conspiracy to commit murder and other alleged offenses against miners who have not joined the strike. It gave no further details.

Union spokesman Marcel Golding described the Klerksdorp raid as "an attempt to break the strike and to undermine legitimate trade union activities."

The statements came on the third day of the strike, which has seriously disrupted gold and coal mining, the backbone of the South African economy.

It said those attending the Klerksdorp meeting were told that "radical steps had to be taken to prevent the miners returning to work."

{Union leader Cyril Ramaphosa said the union had sent lawyers to Klerksdorp and was filing an urgent court application to get the men released, The Associated Press reported. Of the murder conspiracy charges, he said, "It would be pretty stupid to be talking about murder at a union meeting."}

Golding said the strike was expected to spread tonight to the Rand Refinery in a Johannesburg suburb, where the country's gold bars are produced. Golding said 120 of the 150 black staff at the refinery had voted in a ballot last night to join the strike.

Today's arrests coincided with an ultimatum from one of the affected gold mines, Loraine, in the Orange Free State, that it would dismiss about 2,500 miners if they did not report for today's night shift.

The owners, Angloval Mining Co., said they were acting because the union is not recognized at Loraine and the strike there was illegal under South African law.

A company statement accused the strikers of using intimidation.

The strike, called to back demands for a 30 percent wage increase, is seen as a significant test of strength between emergent black unions and the white authorities.

The country's biggest antiapartheid organization, the United Democratic Front, today declared its support for the miners.