South Africa's Winnie Madikizela-Mandela, an anti-apartheid stalwart and wife to Nelson Mandela when he was imprisoned on Robben Island, died on Monday, her personal assistant Zodwa Zwane said. She was 81.

"She died after a long illness, for which she had been in and out of hospital since the start of the year," Victor Dlamini said in a statement.

"She succumbed peacefully in the early hours of Monday afternoon surrounded by her family and loved ones."

"She fought valiantly against the apartheid state and sacrificed her life for the freedom of the country," the statement said.

"She kept the memory of her imprisoned husband Nelson Mandela alive during his years on Robben Island and helped give the struggle for justice in South Africa one its most recognisable faces."

Madikizela-Mandela was jailed several times for her part in the fight against white-minority rule and she campaigned for the release of her husband at home and abroad.

But her marriage to Mandela began to fall apart in the years after he was released from prison in 1990. The couple divorced in 1996, nearly four decades after they were married. They had two children together.

Then ANC Womens League President Winnie Madikizela-Mandela smiles during the closing rally of the 51st National Conference of the ruling ANC in Stellenbosch

Winnie Madikizela Mandela, ex-wife of former South African president Nelson Mandela, gestures to supporters at the 54th National Conference of the ruling African National Congress (ANC) at the Nasrec Expo Centre in Johannesburg

Winnie Madikizela Mandela, ex-wife of former South African president Nelson Mandela, smiles as she arrives for the 54th National Conference of the ruling ANC at the Nasrec Expo Centre in Johannesburg

Winnie Madikizela Mandela, former wife of Nelson Mandela, casts her vote during the South African municipal elections in Soweto