A violent protest broke out on Friday at a Johannesburg store run by Walmart's South African unit Massmart, according to a Thomson Reuters witness.
The store is part of Massmart's Makro chain, whose employees have been on a wage strike for a week.
Marune Rossouw, a Thomson Reuters employee, said she was trapped inside the store with around 50 other people as protesting staff rampaged through the car park, throwing stones at cars.
"We're trapped inside, in the bottles section, and the guys are outside toyi-toying (dancing)," she said by phone. "They've told us to move to the back of the store."
A police spokesman could not immediately confirm the incident at the store, in a middle-class northern suburb of Johannesburg.
A Massmart spokesman also could not immediately offer confirmation.