South Africa's Winnie Madikizela-Mandela, an anti-apartheid campaigner and former wife of the late Nelson Mandela, has died aged 81.

Key points: She died after a long illness, but attended Easter services over the weekend

She died after a long illness, but attended Easter services over the weekend She and her husband were a symbol of the anti-apartheid struggle for decades

She and her husband were a symbol of the anti-apartheid struggle for decades Her activism was marred by criminal convictions and allegations of corruption

She died in a Johannesburg hospital, following a long illness that had kept her in and out of hospital since the start of the year, her family said in a statement.

"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."

South African President Cyril Ramaphosa led the outpouring of grief for Ms Madikizela-Mandela, paying tribute to her in front of a crowd of hundreds gathered outside her home in Soweto.

"Winnie Mandela ... leaves a huge legacy and, as we say in African culture, a gigantic tree has fallen," he said.

"She has been one of the strongest women in our struggle, who suffered immensely under the apartheid regime, who was imprisoned, who was banished, who was treated very badly.

"Winnie Mandela had a great impact on the African continent. She is fondly remembered as a very gallant and brave person."

Locals of the Soweto neighbourhood and supporters of the ruling African National Congress (ANC) sang songs and danced in the rain as they remembered Ms Madikizela-Mandela.

Sorry, this video has expired Mourners pay tribute to Winnie Madikizela-Mandela

"It is very sad that today we lost a stalwart, who is an inspiration, who is still an inspiration even after her passing to all the women in South Africa," ANC supporter Wendy Makhoba said.

"We are here to mark this saddest moment in our history.

"As young people, we grew up in this part of town and I know everywhere else in Africa [people will] say thanks to mama Winnie Mandela for the role she played in our struggle," Themba Mbhele said.

"She is our mother, she was there when things were tough."

Together, Mandela and Ms Madikizela-Mandela were a symbol of the anti-apartheid struggle for nearly three decades.

Nelson Mandela walked out of prison in February 1990 with his wife Winnie by his side. ( Reuters )

But Ms Madikizela-Mandela's activism was marred by convictions for kidnapping and assault and allegations of corruption, and she was jailed several times for her part in the fight against white-minority rule.

During her husband's 27-year incarceration, Ms Madikizela-Mandela campaigned for his release and for the rights of black South Africans, suffering years of detention, banishment and arrest by the white authorities.

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In 1977, she was banished to a remote town, Brandfort, where neighbours were forbidden to speak to her. She was banned from meeting with more than one person at a time.

The woman who returned to Johannesburg in 1985 was much harder, more ruthless and bellicose, branded by the cruelty of apartheid and determined vengeance.

In an infamous 1986 speech she threatened "no more peaceful protests". Instead, she endorsed the "necklacing" method of killing suspected informers and police with fuel-doused tires put around the neck and set alight.

Ms Madikizela-Mandela suffered years of detention and arrest during her husband's imprisonment. ( AP Photo, file )

From 'mother' to 'mugger' of new South Africa

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For Ms Madikizela-Mandela, 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.

As evidence emerged in the dying years of apartheid of the brutality of her Soweto enforcers, the "Mandela United Football Club" (MUFC), her nickname switched from "Mother of the Nation" to "Mugger".

Blamed for the killing of activist Stompie Seipei, who was found near her Soweto home with his throat cut, she was convicted in 1991 of kidnapping and assaulting the 14-year-old because he was suspected of being an informer.

Her six-year jail term was reduced on appeal to a fine.

Appearing at the Truth and Reconciliation Commission (TRC) set up to unearth atrocities committed by both sides in the anti-apartheid struggle, Ms Madikizela-Mandela refused to show remorse for abductions and murders carried out in her name.

Only after pleading from anguished TRC chairman, Nobel laureate and former archbishop Desmond Tutu, did she admit grudgingly that "things went horribly wrong".

In her later years, Ms Madikizela-Mandela had frequent run-ins with authority that further undermined her reputation.

'Defining symbol' of anti-apartheid fight

Nobel laureate and former archbishop Desmond Tutu described her as "deeply inspirational." ( AP Photo, file )

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Despite this, Ms Madikizela-Mandela remained a venerated figure in the ANC, which has led South Africa since the end of apartheid in 1994.

The ANC, which was the main movement against apartheid, had lost popularity in recent years in part because of scandals linked to former president Jacob Zuma, who resigned in February.

Mr Tutu, a periodic critic of the ruling party, noted her passing by describing Ms Madikizela-Mandela as "a defining symbol" of the fight against apartheid.

"She refused to be bowed by the imprisonment of her husband, the perpetual harassment of her family by security forces, detentions, bannings and banishment," he said.

"Her courageous defiance was deeply inspirational to me, and to generations of activists."

Ms Madikizela-Mandela had been in and out of hospital since the start of the year, according to her family. She had back surgery a year ago.

She had attended Easter services in Soweto over the long weekend.

The family said it will release details of her memorial and funeral services when they are finalised.

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Reuters/AP