Nelson Mandela's is in a critical condition, South Africa's presidency said , a sudden and significant deterioration in the hospitalised hero's health.
"The condition of former president Nelson Mandela, who is still in hospital in Pretoria, has become critical," said presidential spokesman Mac Maharaj, 16 days after Mandela was admitted for treatment for a lung infection.
Mandela, now a frail 94-year-old, was previously said to be in a serious but stable condition.
But after more than two weeks of intensive treatment at Pretoria's Mediclinic Heart Hospital, his condition is said to have deteriorated over the weekend.
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."
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 for years.
Throughout most of the last week his condition was said to be improving, and there had been suggestions from his family that he may be released.
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.
The two also met with Mandela's wife Graca Machel at the hospital and discussed the condition of the Nobel Peace laureate.
Zuma appealed to South Africans and people worldwide to pray for Mandela, his family and the medics attending to him "during this difficult time."
The reaction was swift. Many South Africans took to social media to express their wish that Mandela get well, or at least be at peace.
Reverend and former government official Frank Chikane described the latest reports "worrying." "Must pray harder, assure family of (your) support," he urged compatriots via Twitter.
In Washington the White House National Security Council offered its thoughts and prayers to Mandela, his family and people of South Africa.
As much as Mandela is loved by South Africans, with the latest hospitalisation many have come to terms with their hero's fragility and have begun to look again 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.
"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 been indicating.
US news channel CBS had at the weekend given details of failing organs and said that Mandela was "unresponsive" and "has not opened his eyes for days". It claimed Mandela's liver and kidneys were operating at 50 percent of their capacity.
But authorities had refused to comment on the speculation.
It also emerged that the military intensive care ambulance that rushed Mandela to hospital in the early hours of June 8 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 (30 miles) 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.
The African National Congress said it "has noted with concern" that Mandela's condition had worsened.
"The African National Congress joins The Presidency in calling upon all of us to keep President Mandela, his family and his medical team in our thoughts and prayers during this trying time."
Mandela's grandchildren based in the US also encouraged prayers in a tweet shortly after the news that their grandfather had become critical.
"Let us never forget to pray. God lives. He is near. He is real. He is not only aware of us but cares for us. He is our Father," said Swati Dlamini and Zaziwe Dlamini-Manaway, tweeting under the handle @BeingMandela. - AFP, June 24, 2013.