Nelson Mandela was critically ill in hospital on Monday after his condition suddenly deteriorated, leaving South Africans anxiously awaiting the latest news of their revered anti-apartheid icon.

"The condition of former president Nelson Mandela, who is still in hospital in Pretoria, has become critical," presidential spokesman Mac Maharaj said in a statement late Sunday. The frail 94-year-old was admitted to hospital over two weeks ago, in the early hours of June 8, for treatment for a lung infection.

But after intensive treatment at Pretoria's Mediclinic Heart Hospital and some signs of improvement, his condition deteriorated. President Jacob Zuma visited Mandela on Sunday evening and was told by doctors "that the former president's condition had become critical over the past 24 hours."

Maharaj told AFP on Monday there was no update on this condition overnight.

Mandela, who became South Africa's first black president in 1994, is due to celebrate his 95th birthday on July 18.

He has been hospitalised four times since December, mostly for the pulmonary condition that has plagued him since his time in an apartheid jail. Zuma sought to assure the country that medics were now doing all they could to save his life.

"The doctors are doing everything possible to get his condition to improve and are ensuring that Madiba is well-looked after and is comfortable. He is in good hands," Zuma said, using the revered leader's clan name. Zuma was accompanied to the hospital by the ruling ANC deputy president Cyril Ramaphosa.

At the hospital, the two also met with Mandela's wife Graca Machel, who has been by Mandela's bedside since he was taken ill.

Zuma appealed to South Africans and people worldwide to pray for Mandela, his family and the medics attending to him "during this difficult time."

Flowers, cards, balloons and messages of support were left outside the gate of Mandela's Pretoria hospital.

"Two weeks in hospital is really a long time for an old man. I'm worried, we are all worried," said Mthandeni Mjwara in Mthatha, a rural town in the region where Mandela grew up.

"It's quite sad to hear that his health is not improving, I was really hoping for good news." In Washington, the White House said its thoughts and prayers were with Mandela, as US President Barack Obama prepares to visit South Africa.

"We have seen the latest reports from the South African government that former president Mandela is in critical condition," National Security Council spokeswoman Caitlin Hayden said. "Our thoughts and prayers are with him, his family and the people of South Africa."

Obama leaves Wednesday on a tour of Africa that will take him to Senegal, South Africa and Tanzania.

The possibility of a meeting between the first black presidents of both South Africa and the United States has been hotly anticipated for years. The White House has said it will defer to Mandela's family regarding any meeting.

As much as Mandela is loved by South Africans, with the latest hospitalisations many have come to terms with their hero's fragility.

"It is not up to us to decide what happens now. There is nothing we can do," said Aphiwe Ngesi a teacher in Mthatha.

"All we can do is hope for the best."

Many South Africans have instead begun to look anew at his achievements

During 27 years in jail he became the figurehead of the anti-apartheid movement. On his release he negotiated an end to white rule and won the country's first fully democratic elections.

As president he guided the country away from internecine racial and tribal violence.

It was 18 years ago today that, in a deeply symbolic moment, he handed the rugby world cup to a victorious Springboks captain Francois Pienaar.

The impact of a black president appearing at this most white of South African sports still reverberates today.

"Mandela soared above the petty confines of party politics," said political commentator Daniel Silke.

Sunday's announcement came after unconfirmed media reports that Mandela's condition was worse than what authorities and relatives had suggested.

Mandela's daughter Makaziwe Mandela rubbished those claims telling CNN earlier Sunday that "he still opens his eyes, ...the touch is there."

The South African government has also been criticised amid revelations that the military ambulance that carried Mandela to hospital developed engine trouble, resulting in a 40-minute delay until a replacement ambulance arrived.

The presidency said that Mandela suffered no harm during the wait for another ambulance to take him from his Johannesburg home to a specialist heart clinic in Pretoria 55 kilometres away.

"There were seven doctors in the convoy who were in full control of the situation throughout the period. He had expert medical care," said Zuma.

The doctors also dismissed the media reports that Madiba suffered cardiac arrest (the day he was taken to hospital). "There is no truth at all in that report," said Zuma.

News about the icon's health made headlines in most local newspapers: "World on edge as Mandela's condition worsens" said Business Day.