Walus, 69, a Polish national who emigrated to South Africa in 1981, was granted parole by the Constitutional Court last week after serving nearly 30 years of his life sentence for Hani's murder.

He was expected to be released this week, but was stabbed by a fellow prison inmate on Tuesday and is undergoing treatment. His parole will be finalised after medical clearance is granted, the justice ministry said.

The decision to grant Walus parole has been met with fierce criticism and protests in South Africa, where Hani is remembered as a hero in the fight to end white minority rule.

Demonstrators in Pretoria marched to the prison where Walus is being held on Wednesday holding signs that read: "Don't kill Chris Hani again." Hani's widow, Limpho Hani, gave an emotional interview on television calling the judgment "diabolical".

"There's a public outcry over this decision. People feel like the constitution has failed them because it's letting go of someone who has murdered their beloved hero, beloved activist," said constitutional law expert Ropafadzo Maphosa.

However, she said, Walus was legally eligible for parole.

Hani was both a senior member of the African National Congress (ANC) and the head of the South African Communist Party (SACP) when he was gunned down outside his home in Johannesburg.

His assassination triggered nationwide riots which ultimately precipitated the transition out of apartheid, said historian Tshepo Moloi.

"Because of the death of Chris and the way he died, within less than a year South Africans of all races for the first time went to the polls," Moloi said.

South Africans are angry to see Hani's killer released on parole on principle, but the outcry also speaks to broader frustrations about the lack of progress in ending racial and economic inequality, he told Reuters.

"It's now almost 30 years since democracy, yet the majority of black people are still living in squalor. And the very people who they fought against, the majority of them are well-off," Moloi said.

"(People) put these things together and say... is this the South Africa that we fought for? No. Clearly."

(Reporting by Catherine Schenck and Shafiek Tassiem; Writing by Nellie Peyton; Editing by Nick Macfie)