The South African Communist Party (SACP) members on Saturday marched in Pretoria central to commemorate the death of former secretary general Chris Hani.
"Today's protest forms part of [the] activity for the 21st anniversary of comrade Hani's death," said SACP spokesperson Alex Mashilo.
Hani was shot dead in the driveway of his home in 1993.
Mashilo said more than 2 500 members marched to the Kgosi Mampuru correctional facility to demand that Hani's killer Clive Derby-Lewis remain behind bars.
"We believe that he [Derby-Lewis] is a convicted murderer, who has no remorse and is unrehabilitated," said Mashilo.
"Up until today, he has only told the partial truth about [the] assassination of Hani."
The march began around 10am and dispersed by the afternoon.
Attacked in prison
Derby-Lewis, who was sentenced to 25 years behind bars, has already served more than 20 years of his sentence. The 78-year-old man, who was reportedly diagnosed with cancer, has twice been denied medical parole.
He first made an application in 2012, and another in 2011.
Derby-Lewis was attacked twice this year in prison.
In February, he was attacked with a spoon by another prisoner, and suffered lacerations on the head and hand.
In March, a prisoner attacked him with a piece of glass and he sustained a wound in his back. He was taken to hospital for treatment. – Sapa