Striking truck drivers in the Johannesburg CBD threw stones at passing trucks on Tuesday, metro police said.
"The trucks were passing Beyers Naude square, where truck drivers are protesting," Chief Superintendent Wayne Minnaar said.
"Companies are requested to tell their people not to send any trucks into the Joburg CBD in order to avoid further violence and damage to trucks."
Earlier, the SA Transport and Allied Workers' Union (Satawu) said over 20 000 employees in the road freight sector were on strike over their pay increase on Tuesday.
Employees in the road freight sector - which includes truck drivers - were demanding a 12 percent increase. Their employers had offered 8.5 percent.
Satawu said the strike came about when wage negotiations deadlocked after protracted discussions since early June at the National Bargaining Council for the Road Freight and Logistics Industry (NBCRFLI).
Satawu spokesman Vincent Masoga said no marches were planned for Tuesday, but that workers would gather at Beyers Naude Gardens, opposite the Johannesburg Library, for news.
Notice was given for a strike from Monday, but the effects were not immediately visible because it was a public holiday.
Minnaar said traffic was being diverted from Simmonds street and Pritchard street as well as at the intersection of Harrison and Pritchard streets and President and Sauer streets.
Satawu, a Congress of SA Trade Unions affiliate, is the biggest union in the four-union strike, with an estimated 28,000 members in the road freight sector, said Masoga.
Members ranged from drivers delivering fuel to workers associated with a truck network which travelled around the country or crossed borders into neighbouring countries for other deliveries.
The other unions are the Transport and Allied Workers' Union, the Professional Transport and Allied Workers' Union SA and the Motor Transport Workers' Union.
The Road Freight Association, which represents employers, told its members on its website: "Although unions are consulting members on the proposal, the strike notice stands, so employees can strike..."
Masoga said the parties in the bargaining council were expected to meet at the Commission for Conciliation, Mediation and Arbitration at 3pm on Tuesday for further negotiations.
Satawu said the unions were aiming for an inflation-related wage settlement of 12 percent across the board, for implementation in 2013 and 2014.
They also wanted an equal increase for workers classified under the council's extended bargaining unit.