“The Mangaung Correctional Centre has an impeccable track record in maintaining the safety of both inmates and employees,” G4S Africa head Andy Baker said in a statement.
“However, the nature and frequency of the incidents that the centre has recently experienced is highly irregular.
“The increase and motive for the attacks on our employees are being investigated through the proper channels.”
He said the situation at the prison was calm, following a hostage situation and a number of assaults on G4S employees earlier in the week.
G4S had asked retired Free State head of security Tatalo Setlai to support the prison as an “operational adviser”.
Baker said on Thursday that a prison guard had been stabbed at the prison. He said there had been stabbings at the prison every day this week, but could not say how many.
On Wednesday, four inmates held a G4S official hostage for 13 hours at the prison.
She was released when the prisoners were overpowered by a special police task force.