Nelson Mandela's family paid tribute to anti-apartheid activist Albertina Sisulu on Friday, saying she had been not only a struggle hero but a mother to Mandela's children when he was in jail.

Sisulu, a veteran of the fight against white-minority rule and a close friend of Mandela's, died Thursday night at her home in Johannesburg. She was 92.

The Mandela family recalled the role "Mama" Sisulu played in both the anti-apartheid movement and their own lives when Mandela and her husband, the late African National Congress (ANC) leader Walter Sisulu, were jailed on charges of trying to overthrow the apartheid regime.

"She has been the pillar of the movement during the most difficult time of our struggle when her husband and many others were languishing in jail," the family said in a statement.

"It is a well-known fact that the Sisulus and the Mandelas share a strong bond that spanned generations. They do not share these ties only because of their common dedication to the struggle for liberation but also because Evelyn Mandela (Nelson Mandela's first wife) and Walter Sisulu are cousins.

"It is these family ties that saw Mama Sisulu being the primary guardian and caregiver of Evelyn Mandela's children during the long period of Nelson Mandela's incarceration."

The family called Sisulu "the mother of our struggle for freedom in South Africa".

Sisulu and Mandela, who is himself 92, remained close after her husband's death in 2003.

She was among the first to visit the former president and Nobel Peace Prize winner when he fell ill with a respiratory infection in January and was hospitalised for two days.