More than 30,000 workers in South Africa's auto industry vowed Tuesday to intensify an nationwide wage strike that has crippled a multi-billion dollar export market.

In the coastal city of Port Elizabeth, dubbed the "Detroit of Africa," thousands of workers took to the streets singing revolutionary songs outside General Motors, Ford and Volkswagen plants.

"The strike still proceeds," said Mphumzi Maqungo, the treasurer of the National Union of Metalworkers (NUMSA), vowing to "intensify" the stoppage.

The workers are demanding a 14-percent pay increase. Employers have offered eight percent.

No talks have taken place between seven major car makers and workers since Monday, when tens of thousands downed tools.

NUMSA vowed to "take the battle to the employers' doorsteps in the form of marches and demonstrations."

Ayanda Madlozi, a worker protesting outside the VW factory in Port Elizabeth, demanded "a wage which would take care of (his) whole family, not peanuts."

Meanwhile at Ford's plant in Mamelodi, on the outskirts of Pretoria, hundreds of workers clad in red T-shirts and berets chanted and danced.

The strike is costing South Africa -- the continent's largest car producer -- around 3,000 vehicles or $60 million at day in lost revenue according to industry lobby group the National Association of Automobile Manufacturers of South Africa.

The group has warned lost production will need to be regained in order to meet obligations to international customers.

"Once the strike is over, which hopefully will be soon, manufacturers will inevitably and invariably take steps to recover part of the lost production," said the group's Nico Vermeulen.

Mercedes Benz, which has 1,500 hourly paid workers on strike, expressed concern about meeting orders worth hundreds of millions of dollars for its sleek C-Class range.

"We have to meet our international obligations," said Mayur Bhana, a Mercedes Benz spokesman.

The South African car industry exports to 148 countries worldwide, with the United States the number one destination, followed by the European Union and other African countries.

Around 60 percent of production is sold outside the country and accounts for around six percent of the country's economy and roughly 12 percent of its exports.

Some of the world's leading brands -- including Toyota, Volkswagen and Mercedes Benz -- have production plants in South Africa.

But the latest work stoppage has provided another beating for investor confidence.

The country is still reeling from the devastating effects of last year's mining industry labour unrest that claimed dozens of lives.

"The very strong nature of South Africa's labour movement is a deterrent to international investment and growth," said GM spokeswoman Denise van Huyssteen.

Economists say the motor industry strike is, however, unlikely to last long as the industry would want to remain on friendly terms with the government given it is protected by tarrifs and duties.

"The automakers are likely to capitulate to workers' wage demands sooner than (later)," said Loane Sharp, a labour analyst with Adcorp, an employment services firm.

Despite being the largest economy on the continent, South Africa is underperforming compared to other fast-growing countries in the region, expanding only by 0.9 percent in the first quarter.

Last year the country registered a meagre 2.5-percent growth rate, hit by the effects of the post-2008 global financial meltdown and the eurozone recession.

National elections are due next year.

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