More than 45 truck drivers were arrested on Thursday as the national strike entered its fourth day, Gauteng traffic police said.

Several people have been injured in incidents countrywide since the protest action started on Monday.

"Charges include illegal marching... and public disturbance. It resulted in a serious accident that occurred an hour ago," said spokeswoman Chief Bufaphi Nxumalo.

"We are still going to continue arresting them, especially if they are disturbing the public and marching without permission."

About 20 of those arrested were taken into custody on the R21 in Ekurhuleni, east of Johannesburg.

About 20,000 workers in the road freight transport sector have been on strike over wages since Monday.

The R21 and R24 near OR Tambo International Airport were reopened to traffic early on Thursday afternoon after striking truck drivers forced its closure by throwing stones and burning trucks, metro police said.

"Motorists are advised to be extra careful," said Ekurhuleni metro police spokesman Inspector Kobeli Mokheseng.

He said police believed criminal elements had infiltrated the strike.

Four people have been reported injured due to violence by striking truck drivers.

Three people were injured near the R21 around 7am when strikers blocked the road and threw stones at trucks.

Ekurhuleni EMS spokesman Rogers Mamaila said a truck driver not on strike broke his leg following an accident caused by striking drivers on Thursday morning.

He said the truck was on Voortrekker Road, travelling away from OR Tambo International Airport around 8.45am when striking drivers began throwing stones and bricks at the truck.

In attempting to avoid them, the man overturned the truck, which caught fire.

"The... incidents of strikers is so spread out, we might not know about all incidents. I know private ambulances have been attending [to] others," said Mamaila.

SA Transport and Allied Workers Union (Satawu) spokesman Vincent Masoga said the union were still investigating reports of violence in Ekurhuleni.

In the Western Cape, strikers had gathered near Cape Town International Airport, but there had been no reports of violence.

"The strikers... near the airport are calm. There are no reports of violence or injuries," said Captain FC van Wyk.

Police were on hand to deal with the situation, he said.

Unions have reverted to a 12 percent pay demand after rejecting a lower offer tabled by employers on Tuesday.

The Road Freight Employers' Association (RFEA) had proposed a staggered increase of 8.5 percent effective from March, and a further 0.5 percent from September next year.

Masoga said Satawu sympathised with the public if shortages of goods were experienced as a result of the strike.

MC Langbrecht, chairman of the SA Petroleum Retailers Association, said there was no reason to panic regarding petrol supply.

"It is early days. We went through a similar situation last year and it took a lot longer. There is no reason to panic," he said.

"The contingency plans from the companies are in place, and updated, and focused on a daily basis."

Langbracht said from time-to-time, a specific station might not have supply, but overall the situation was under control.

On June 13, the Constitutional Court ruled Satawu was responsible for damages caused during a march by security guards in Cape Town, in May 2006.

A similar strike by road freight unions in February 2011 left at least 20 trucks damaged, 16 people injured and scores of arrests as non-striking truckers were beaten and their trucks looted and burned.

On May 25 2010, the Passenger Rail Agency of SA (Prasa) blamed striking Satawu train workers for the torching of four train coaches.

On April 14 2009, police fired rubber bullets at striking truck drivers who assaulted two men in Johannesburg.

On the same day, a group of strikers also stoned and looted a delivery van on the corners of Biccard and De Korte Streets in Braamfontein, Johannesburg.

On April 8 in the same year, a truck driver was seriously injured in Port Elizabeth when striking workers stoned and petrol-bombed his truck.