Fires started by awaiting-trial prisoners at the Grootvlei prison outside Bloemfontein have damaged a cell and a clinic during a protest.
Free State police spokesman Brigadier Billy Jones said police were investigating a case of arson.
Awaiting-trial prisoners at Grootvlei refused to go to court this week as part of a protest over prison conditions. They claimed they were being subjected to overcrowding, poor treatment, and racism.
The protest begin last Friday, with sporadic flare-ups this week.
Jones said prisoners also set alight material in a cell on Monday.
Police used rubber bullets to restore calm. A search of the cells recovered items such as cellphones and drugs.
Prison officials, the departments of correctional services and justice and the police were meeting on Tuesday to discuss the matter.
Free State correctional service personal were not immediately available to comment.
Jones said police would stay on standby at the prison to assist prison officials.
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