While petrol suppliers and shops said "contingency plans" would prevent the disruption of services, the South African Transport and Allied Workers Union (Satawu) said the strike would continue indefinitely.

Satawu spokesman Vincent Masoga condemned the violence but said the strikers had no intention of going back to work soon.

"It's going to go on as long as employers are ready to negotiate."

The strike began quietly on Monday with hopes it would be averted after the Road Freight Employers Association offered an 8.5% wage increase at the weekend.

Spokesman Magretia Brown-Engelbrecht said unions had been asking for 9% since September and the offer was made with the intention of avoiding a strike. The unions who had been asking for 9% then wanted 12%, she explained.

"We view the unions' conduct in extreme bad faith.

"As a result, we now face prolonged strike action and accompanying financial losses," Brown-Engelbrecht said.

Masoga said the strikers had been asking for 12% since June.

While the deadlock continues, there were reports of intimidation.

Mark Basson, from Pinetown in KwaZulu-Natal, wrote on Facebook: "Whew, was in Pinetown by the McDonalds, when strikers started stoning a truck behind me, unfortunately the truck driver was knocked unconscious with his foot still on the accelerator. I tried to stop the runaway truck by hanging on and steering, it drove onto a grassy park and I managed to steer it into a tree. The driver suffered a broken jaw."

In Cleveland, east of Johannesburg, a burnt-out truck with slashed tyres, lay on the side of the road as a warning to trucks whizzing by.

In Cape Town, two trucks were set alight in Nyanga township, Sapa reported.

Trucks with smashed windows zigzagged through the streets of Johannesburg and striking workers stoned trucks to prevent them from entering the Durban harbour.

A cash-in-transit industry source said: "If this progresses, money might not get to ATMs by Friday. This particular level of violence in truck strikes is unusual."

But, in Johannesburg, strikers marched peacefully from Beyers Naude Square to Park Station before knocking over a gate and dispersing. A march of 500 workers to the Durban City Hall was also peaceful.

The Consumer Goods Council expressed concern about the wage deadlock "in light of the pending festive period".

The South African Petroleum Industry Association said that some stations could run out of certain grades of fuel but should not run completely dry.