The union is demanding a 15 percent wage increase, well above inflation which was at 4.2 percent in July.

"We are having demonstrations at plant levels in the Eastern Cape, Gauteng and KwaZulu Natal," union spokesman Castro Ngobese said.

NUMSA has asked an industry group - which represents seven companies including Toyota Motor Corp, Ford Motor, Volkswagen AG and General Motors Co - for a one-year agreement guaranteeing a 15 percent wage rise.

The other companies are Nissan Motor, BMW and Daimler AG's Mercedes-Benz division.

"The employers should come on board because we have been negotiating with them for a period of more than two months now. Nothing has come out concretely out of those negotiations," said Ngobese.

The South African unit of German carmaker BMW said its local plant was closed as of Wednesday until the strike was over.

 "There is no way we can build cars without workers. There is no contingency plan in place," said Guy Kilfoil, a BMW spokesman in South Africa, adding he hoped the strike will be resolved soon through the Automobile Manufacturers Employers Organisation industry group.

Other carmakers and the industry body were not immediately available for comment.

Ngobese said about 6,000 workers in tyre manufacturing are not going to strike.

A strike could have a big impact on a South African auto industry that is already struggling to remain competitive.

Motor vehicle production in South Africa is expected to hit 443,000 units in 2010 from 374,000 last year, according to the National Association of Automobile Manufacturers.

South Africa's government sees auto manufacturing as a key industry to help boost growth and jobs and has tried to help carmarkers build up exports.

Separately, more than a million South African public sector workers staged a one-day strike on Tuesday, threatening an extended labour stoppage this week.

The unions have set a Thursday deadline for the government to reach a deal.