Police Minister Nathi Mthethwa said on Thursday striking truck drivers who committed violence and broke the law would be acted against by law enforcement.

"We are monitoring the situation and frankly, what we have been observing is pure criminality," said Mthethwa in a statement.

He said acts of violence and intimidating innocent people could not be justified as part of public protests, and police intend to act harshly against such lawlessness, with arrests imminent.

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

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.

The Congress of SA Trade Unions on Thursday urged strikers to avoid violence.

Spokesman Patrick Craven said in a statement that while Cosatu fully supported striking unions, with Cosatu-alligned SA Transport and Allied Workers Union included, it appealed to workers to not resort to violence, even when provoked.

The trade federation said it hoped employers would return to the negotiating table as soon as possible.

The Democratic Alliance condemned the violence linked to the road freight strike in Ekurhuleni on Thursday.

Gauteng leader John Moodey said in a statement the on-going violence was not acceptable, and strikers and union members needed to be held accountable for their actions.

The party called on the provincial safety and security MEC to take swift action before further damage was done.

"The safety of non-striking workers and the community at large is being placed at risk, and police need to ensure their protection," said Moodey.

While the DA recognised the democratic right to strike, strikes could not be conducted in a violent and destructive manner.

Mthethwa said police did have a responsibility to police public protests and gatherings, but the Gatherings Act left much of the emphasis with convenors or organisers to ensure such events were conducted peacefully.

"Any contravention of this must result in the organisers facing criminal charges," Mthethwa said.

He said allocating police resources for public protests and demonstrations was redirecting focus away from priority crimes such as murder and sexual offences.

"That is where all our energies must be directed at," said the minister.

Satawu spokesman Vincent Masoga said: "We are hoping the employers will come back today [Thursday], otherwise we will continue the strike."

RFEA spokeswoman Magretia Brown-Engelbrecht said: "We have committed ourselves to being available for further talks. Nothing has been forthcoming thus far and we shall see where we go."

The RFEA was hopeful that between Thursday and Friday, the negotiation process would resume, she said.

The unions involved are Satawu, the Professional Transport and Allied Workers' Union SA, the Transport and Allied Workers' Union of SA, and the Motor Transport Workers' Union.