Uncompromising opponent of racial-segregation system maintained ties with Nelson Mandela despite their divorce in 1996.

Winnie Mandela, the ex-wife of the late South African anti-apartheid icon Nelson Mandela, has died at the age of 81.

A veteran herself of the anti-apartheid struggle, Winnie Mandela died on Monday afternoon, her personal assistant said.

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

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

Born in the Eastern Cape's Bizana town in 1936, Nomzamo Winifred Madikizela moved to Johannesburg in 1953 to pursue studies in social work before meeting Nelson Mandela, her future husband who was then an anti-apartheid campaigner.

Winnie Mandela developed a reputation as an uncompromising opponent of the then-predominant racial segregation system.

Winnie Mandela, centre, was for long a powerful figure in the governing ANC [AP]

Al Jazeera's Catherine Soi, reporting from Johannesburg, said her family described her as a strong woman who relentlessly fought for the majority black population's rights. 

"Her family gave a very moving tribute about how they said she really is the face of the struggle against apartheid," she said.

"She wasn't married very long to Nelson Mandela before he was imprisoned, so she was very much his face and his voice to the outside world. And she took that role very seriously." 

Winnie Mandela was married to Nelson Mandela from 1958 to 1996, although for almost three decades of that time they were separated due to his imprisonment.

Despite their separation two years after his release in 1990, and their divorce in 1996, she kept his surname and maintained ties with him.

Winnie Mandela kept Nelson Mandela's surname despite their 1996 divorce [AP]

Late on Monday, South African President Cyril Ramaphosa visited the Soweto home of Winnie Mandela and paid tribute to her.

"The death of Winnie Madikizela-Mandela is a great loss in that she has been one of the strongest women in our struggle who suffered immensely under the apartheid regime," he said.

"She remained strong, she remained determined, she was courageous and in many ways she was also very stubborn.

"Stubborn on behalf of our people because she knew that out of stubborn disposition she would be able to inspire millions of South Africans."

A memorial service will be held on April 11 and a national official funeral on April 14, Ramaphosa said.