The authorities in the South African city of Durban have freed 14 workers who had been “locked up in a [mask-making] factory since Monday”, a local government minister said.
They also arrested the owner of the business.
The workers “were forced to manufacture hundreds of masks to meet the demand caused by the outbreak of coronavirus”, KwaZulu-Natal’s tourism and environmental affairs minister Nomusa Dube-Ncube said in a statement.
They were “subjected to inhuman conditions”, she added.
Ms Dube-Ncube said that officials were acting on a tip-off from the public after its inspectors found that masks were selling in the shops for $44 (£35) each.
South Africa is currently in the third day of a 21-day lockdown as it attempts to slow down the spread of coronavirus.
There are more than 1,100 confirmed cases in South Africa, higher than any other country on the continent.
One person has died after contracting Covid-19, the disease caused by the virus.