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Criticism of Nelson Mandela is rare in South Africa, much less so when he is lying in a hospital bed. But a few critics are still willing to break the taboo.

The 94-year-old's opposition to apartheid and his role in negotiating a peaceful democratic transition have won him worldwide, but not, it seems, universal adoration.

Twenty years after those talks, some still believe the deal he struck with South Africa's white rulers ensured blacks would be disenfranchised for decades to come.

Amukelani Ngobeni, a youth leader with the black consciousness party the Azanian People's Organisation, is one such critic.

With whites still earning six times more than blacks on average, he recently demanded Mandela apologise before he dies for "selling out black people's struggle".

"Mandela and his friends... could not wait to occupy the global political space at the expense of the struggle for complete political, social and economic emancipation," he said.

The similarly minded Pan Africanist Congress's youth spokesman Sello Tladi also accused Mandela of being a "sell out."

But his party quickly distanced itself from the "reckless" statement made by "cranks" in its youth brigade.

Such back-peddling normally follows anti-Mandela comments as quickly as the public backlash.

In 2010, Mandela's ex-wife Winnie, who he separated from two years after his release from prison in 1990, let loose in an apparent unguarded moment.

"Mandela let us down. He agreed to a bad deal for the blacks," she was quoted as saying.

"Economically, we are still on the outside," she added, according to the article by Nadira Naipaul, wife of Nobel literature laureate V.S. Naipaul.

In the face of public outcry Winnie Mandela denied ever giving the Naipaul interview, and local media speculated she may have let slip in a private visit from the literary couple.

While Mandela was long criticised for his support for violent resistance to apartheid, he has also come in for criticism for his role as president from 1994 to 1999.

Mandela -- already a septuagenarian when he took office -- had expressed doubts about running the country he fought to create.

But while he managed to work with his former white jailers to "avoid a bloody civil war" he was not hands-on in the day-to-day running of the nation.

During his administration, biographer Anthony Sampson later noted, then deputy president Thabo Mbeki "was more decisively running the country as Mandela became increasingly aloof from day-to-day government."

He behaved "more like a constitutional monarch than an executive president."

Still, in Johannesburg's impoverished township of Alexandra, few are willing to criticise a man who is now breathing with the help of life support machines.

Only a highway separates the area from upmarket suburb Sandton, home to Africa's largest stock exchange.

"There are small groups saying he sold us out, but they are a very small minority," said 22-year-old Khetha, a trainee technician.

"He did his work," he said, adding that "blacks still don't have economic freedom, whites are more advantaged. If you compare the life of people from Sandton with those of Alex, it's obvious."

Others also temper their criticism.

"Even if he has done some things wrong, we don't speak about it because everybody idealised him," according to Mark Dons, 46.

"Mandela had his mistakes but, because of the man he was, people overlook them."

The former president's actions were necessary, said Joseph Mulaudzi also 46-years-old.

"He had to compromise on a lot of things during the reconciliation process," he said.

In the early 1990s "there were a lot of tensions ... if it was not for him, we would have had a civil war."

"That's why we can enjoy freedom. There had to be compromise," he added.