South African police fired rubber bullets at rock-throwing farm labourers in the western grape-growing region on Thursday, marking the second straight day of clashes in a country braced for spreading labour unrest.

Hundreds of striking workers seeking higher wages and better working conditions again blocked a major highway running through the town of De Doorns, about 100 kms (60 miles) east of Cape Town, hurling stones at motorists.

The clashes were less violent than on Wednesday, when police arrested more than 40 in areas near De Doorns, one of the biggest vineyard areas for the country's grape export industry.

This week's violence points to little let-up in a wave of labour unrest that began six months ago in the country's platinum mines and then swept through the trucking and agriculture sectors in Africa's biggest economy.

"All the towns are quiet except for De Doorns, which is a headache," said police spokesperson, Mzikayisi Moloi. No injuries or arrests had been reported on Thursday, he said.

Labourers in South Africa are mostly poor blacks who have seen scant improvement in their financial situation since the end of white-minority apartheid rule in 1994.

The farm workers want to double their salary to R150 a day from the R69 minimum wage.

"The workers will not go back to work. They are sick and tired of the farmers' arrogance," said Nelie Barends, a leader of the Bawsi Agricultural Workers Union of South Africa.

In the worst violence since the end of apartheid, police killed 34 strikers at a Lonmin platinum mine last August, tarnishing South Africa's reputation among investors and prompting downgrades of its sovereign debt ratings.

With gold and coal mines employing more than 250 000 people due to begin industry-wide wage talks in coming months, analysts expect labour relations to cast a shadow over an economy forecast to grow by about 3% this year.