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South Africa's Winnie Madikizela-Mandela, an anti-apartheid stalwart and the wife of Nelson Mandela when he was imprisoned on Robben Island, died on Monday, the Mandela family said in a statement. She was 81.

She died in Johannesburg after a long illness, for which she had been in and out of the hospital since the start of the year.

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

"The Mandela family are deeply grateful for the gift of her life and even as our hearts break at her passing, we urge all those who loved her to celebrate this most remarkable woman."

During Nelson Mandela's 27-year incarceration in his fight against apartheid, Madikizela-Mandela campaigned for his release and for the rights of black South Africans, undergoing detention, banishment and arrest herself.

"She dedicated most of her adult life to the cause of the people, and for this was known far and wide as the Mother of the Nation," the family's statement added.

The ruling but beleaguered African National Congress, which has ruled South Africa since Nelson Mandela won the historic 1994 election that overturned racist all-white rule, paid tribute to Madikizela-Mandela.

It is with great sadness to announce the passing of Cde Nomzamo Winnie Madikizela-Mandela today, 02 April 2018. A struggle icon has fallen. #RIPMamaWinnie pic.twitter.com/xIJKDDVOkQ — ANC Parliament (@ANCParliament) April 2, 2018

Just a day before she died, Madikizela-Mandela had celebrated Easter in Soweto, Bishop Gary Rivas told reporters gathered at the Netcare Milpark Hospital.

“An icon, a giant was laid to rest," he said. "We thank god for her life."

But while she was hailed as the mother of the "new" South Africa by some, Madikizela-Mandela's legacy as an anti-apartheid heroine was tarnished by her actions after Nelson Mandela was released.

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Her uncompromising methods and refusal to forgive contrasted sharply with the reconciliation espoused by her husband as he worked to forge a stable, pluralistic democracy from the racial division and oppression of apartheid.

The contradiction helped kill their marriage and destroyed the esteem in which she was held by many South Africans, although the firebrand activist retained the support of radical black nationalists to the end.

In her twilight years, Madikizela-Mandela had frequent run-ins with authority that further undermined her reputation as a fighter against the white-minority regime that ran Africa's most advanced economy from 1948 to 1994.

The end of apartheid marked the start of a string of legal and political troubles that, accompanied by tales of her glamorous living, kept her in the spotlight for all the wrong reasons.