Six trucks and a bakkie were set alight in Mpumalanga on Thursday morning in a protest said to be held by aggrieved mineworkers, police said.
The bakkie and five trucks carrying coal were torched in the Hendrina area, while a truck carrying maize was set alight in nearby Middelburg, police spokesperson Brigadier Leonard Hlathi said.
He said it was understood that Optimum Coal Mine workers were protesting because they had not received salaries for several months.
"They have engaged themselves in unlawful conduct and we cannot condone this. Any grievance you have, you have to handle it in the correct way."
Police were investigating a case of malicious damage to property.
Adiel Hendricks, whose truck was affected, said their driver said burning tyres blocked the road and people were protesting.
The driver had been transporting coal to Arnot Power Station.
"He was pulled out of the vehicle. The keys were pulled out and thrown onto the other side. They forced the drivers to tip their coal loads and started bagging up the coal."
News24 has seen photos of people, who appear to be residents, using maize meal bags, buckets, wheelbarrows and even a filing tray to scoop the coal from the side of the road.
"Forty-five minutes later, the guys had started burning a truck and threatened to assault the drivers."
They apparently proceeded to set all the trucks alight.
It is understood those who started the fires arrived at the scene in a vehicle and were not part of the group protesting on the road.
Hendricks, whose company has been contracted to Eskom for several years, said they had lost three coal carriers in similar incidents over the years and such losses had a knock-on effect for businesses.
"I am past the stages of getting worked up. You can't do anything about it," he said, lamenting on the ability and will of the police to effectively deal with criminal behaviour and arrest those responsible.