PRETORIA/JOHANNESBURG - South Africa's ailing anti-apartheid leader Nelson Mandela is doing much better in hospital, his ex-wife Winnie said on Friday, as US President Barack Obama arrived for a visit that will pay homage to a man he calls his "personal hero".

The faltering health of the first black president of South Africa, a revered symbol of racial reconciliation, has drawn world attention since the 94-year-old was rushed to hospital with a recurring lung infection nearly three weeks ago.

Two-year-old Precious Mali holds a picture of former South African President Nelson Mandela as well-wishers gather outside the Medi-Clinic Heart Hospital where Mandela is being treated in Pretoria June 28, 2013. [Photo/Agencies]

Earlier this week, the government said Mandela's frail condition had turned critical, but since Thursday President Jacob Zuma has reported that his health is improving.    

"I'm not a doctor, but I can say that from what he was a few days ago, there is great improvement," Mandela's ex-wife, Winnie Madikizela-Mandela, told reporters outside Mandela's former home in the Johannesburg township of Soweto.

But, she added, he remained "clinically unwell".

Aboard Air Force One prior to arriving in South Africa, Obama paid tribute to Mandela for the way he led his nation out of apartheid after years of struggle, but said he did not need a "photo op" with the former president.

"Right now, our main concern is with his wellbeing, his comfort, and with the family's wellbeing and comfort," he told reporters before the US presidential aircraft touched down on Friday evening at Waterkloof air force base in Pretoria.

During his weekend trip to Johannesburg, Pretoria and Cape Town, his second stop of a three-nation Africa tour, Obama is scheduled on Sunday to visit Robben Island, where Mandela passed 18 of the 27 years he spent in apartheid prisons.

White House officials have said they will defer to the Mandela family on whether a visit to the hospital to see Madiba, as he is affectionately known, would be appropriate.

Lessons of mandela

Obama told reporters his message in South Africa would draw from the lessons of Mandela's life.

"If we focus on what Africa as a continent can do together and what these countries can do when they're unified, as opposed to when they're divided by tribe or race or religion, then Africa's rise will continue," he said.

White House officials said Obama would hold a "town hall meeting" on Saturday with youth leaders in Soweto, the Johannesburg township known for 1976 student protests against apartheid.

Obama, in office since 2009, is making his first substantial visit to Africa following a short trip to Ghana at the beginning of his first term.

While well-wishers and journalists crowded outside the hospital in the capital Pretoria where Mandela is being treated, a few blocks away, hundreds of demonstrators protested against Obama's visit, some burning US flags.          

Nearly 1,000 trade unionists, Muslim activists and South African Communist Party members marched to the US Embassy shouting slogans denouncing Obama's foreign policy as "arrogant and oppressive".

Muslim activists held prayers in a car park outside the embassy. Leader Imam Sayeed Mohammed told the group: "We hope that Mandela feels better and that Obama can learn from him."

South African critics of Obama have focused in particular on his support for US drone strikes overseas, which they say have killed hundreds of innocent civilians, and his failure to fulfil a pledge to close the US military detention centre at Guantanamo Bay in Cuba housing terrorism suspects.