Johannesburg - The strike over pay and working conditions at dining franchisor Famous Brands [JSE:FBR] in Midrand became unruly on Wednesday, Gauteng police said.
Warrant Officer Mmakgomo Semono said striking workers threw stones at delivery trucks and buses as they arrived to pick up staff at around 16:30.
He said police were called in to calm the crowd of about 200 and no one was injured.
"One guilty person is known. We will be making arrests soon."
Members of the Metal and Electrical Workers Union of SA (Mewusa) went on strike on Monday.
Mewusa union spokesperson, Mametlwe Sebei, said he was informed about the incident by shopstewards on Wednesday night.
Union workers "have been maintaining a high level of discipline" during the strike and the "provocateurs" were former employees of the company.
He said the union condemned unnecessary use of violence.
"We believe that the disengagement of members from work and the solidarity campaign... including the consumer boycott... is enough to bring company to its knees..."
Sebei planned to go the Midrand picket on Thursday morning to "call order" and to make sure "elements of destabilisation" were exposed.
He said the strike would intensify throughout the week and branches in Centurion and KwaZulu-Natal would join within the next couple of days.
"The main demands of the workers concern the recognition of their trade union, Mewusa...," Sebei said on Tuesday.
He said the company, which supplies companies like Steers, Wimpy and Debonairs with sauces, pizza bases and other ingredients, had "derecognised" the union because it would not compromise on demands.
These included improved wages, a new grading system and an end to casual labour and the planned outsourcing of truck drivers.
Workers were lobbying for a boycott of the company's products and planned to picket outside some of their franchises.
Famous Brands could not be reached for comment on Wednesday.
However on Tuesday, Famous Brands CEO Kevin Hedderwick said the company had been in discussions with Mewusa about a "facilitation agreement" but this broke down when they disagreed on Mewusa's demand for plant-level negotiations.
Famous Brands said Mewusa did not represent the majority of the workforce as claimed. Mewusa's representation at the Midrand main support office workforce accounted for 30% and at national level it was 21%.
The company was granted a court order on Monday restraining workers from interfering with management's need to operate the business.
"In management's view the strike is unprotected, as it is based on a defective certificate of non-resolution, relating to collective bargaining issues," said Hedderwick.
The workforce were issued with notices informing them that the principle of no work no pay would apply.
Management were running the business using a combination of contract workers and administrative staff, with no interruptions of services to franchises.
According to Famous Brands' website, between Wimpy and Steers, they sell over six million bottles of tomato and mustard sauce a year through these food outlets.