Janusz Walus, who was about to be released on parole after serving nearly three decades in a South African jail for killing anti-apartheid leader Chris Hani, has been stabbed in prison, the department of correctional services has said.

"A detailed incident report is to be provided at a later stage, but what can be stated at this point is that inmate Walus is stable and... healthcare officials are providing the necessary care," a statement said, which described the stabbing as "unfortunate".

Walus killed Hani in 1993 in a failed attempt to derail South Africa's transition from white-minority rule to democratic rule.

He had been sentenced to death following his conviction in 1993, but it was later commuted to life after South Africa abolished the death penalty.

Hani's widow Limpho condemned the Constitutional Court's recent ruling to release Walus on parole as "truly diabolical".

It has sparked protests in different parts of the country.

Walus is a Polish immigrant whose South African citizenship was revoked in 2017.

The South African government has ruled out deporting him to Poland, saying he would serve his parole in the country.

In prison, he became a symbol for young Polish nationalists and fascists.