"Former president Mandela remains in a critical condition in hospital," South African President Jacob Zuma said in a televised address to an anxious nation broadcast around the globe.
"The doctors are doing everything possible to ensure his well being and comfort," Zuma said.
Mandela, the hero of black South Africans' battle for freedom during 27 years in apartheid jails, was rushed to hospital on June 8 with a recurring lung infection.
Despite intensive treatment at Pretoria's Mediclinic Heart Hospital, the 94-year-old's condition appears to have suddenly and dramatically deteriorated.
Ex-wife Winnie Madikizela-Mandela -- herself a figurehead of the anti-apartheid struggle -- daughters Zindzi Mandela-Motlhajwa and Zenani Mandela-Dlamini and scores of officials flocked to the hospital on Monday.
The family visits, while common since Mandela was admitted 17 days ago, come amid heightened fears for the former statesman's health.
Mandela's eldest daughter Makaziwe has said her father appears to be at peace with himself.
"He has given so much to the world. I believe he is at peace."
At the same time she complained about the "media frenzy" over her father's condition.
"Whether these are the last moments with us, to be with our dad, or there is still a longer (time), but they (media) must back off," she told CNN.
Zuma on Monday also hailed the life of a man seen as the father of the nation, whose citizens must accept his frailty.
"All of us in the country should accept that Madiba is now old," Zuma said, using Mandela's clan name.
"I think what we need to do as a country is to pray for him to be well and that the doctors do their work."
On the world stage Mandela is seen as a moral beacon that continues to shine long after the Nobel Peace laureate retired from public life.
Mandela was last seen in public in 2010 at the football World Cup finals in South Africa.
"He is the father of democracy and this is the man who fought and sacrificed his life," said Zuma, who spent 10 years in jail on Robben Island at the same time as Mandela.
The anti-apartheid hero went on to become South Africa's first black president in 1994 after almost half a century of white minority rule.
'Nothing we can do but to pray for him'
Mandela 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.
As the world looked on, South Africans appeared to be coming to terms with Mandela's decline.
"Unfortunately, there is nothing we can do but to pray for him and the doctors that are helping him," said Phathani Mbath outside the hospital, where flowers, cards and messages of support have piled up.
In Soweto, the township where Mandela lived for more than a decade, James Nhlapo said South Africa must accept Mandela will not live forever.
"There will soon come a time when all the medical help won't work. We have to face that sad reality now," he said as he served customers in his grocery store.
A similar sentiment was expressed in Mthatha, a rural town in the region where Mandela grew up.
"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."
Well wishes have also come from abroad. In Washington the White House said its thoughts and prayers were with Mandela.
US President Barack Obama leaves Wednesday on a much-awaited tour of Africa that will take him to South Africa as well as Senegal and Tanzania.
The White House said it was monitoring Mandela's condition and could not yet say whether his failing health would affect the visit.
Upon his release from jail in 1990 in one of the defining moments of the 20th century, Mandela negotiated an end to apartheid 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 to the day on Monday, in a deeply symbolic moment, Mandela handed the rugby world cup to a victorious Springboks captain Francois Pienaar.
The impact of a black president appearing at this, the most white of South African sporting occasions, still reverberates today.
"Mandela soared above the petty confines of party politics," said political commentator Daniel Silke.
His extraordinary life story, quirky sense of humour and lack of bitterness to his former oppressors has ensured global appeal for the charismatic leader.
The South African government has 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 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.