Nelson Mandela was in a "serious but stable" condition in hospital on Saturday night with a renewed lung infection that has triggered worldwide concern for the South African hero.
Mandela, who turns 95 next month, was whisked to a Pretoria hospital in the early hours of Saturday for his fourth hospital stay in seven months.
"The truth of the matter is a simple one. Madiba is a fighter and at his age as long as he is fighting, he'll be fine," presidency spokesman Mac Maharaj told AFP, using Mandela's clan name.
His wife Graca Machel was at his bedside in hospital after calling off a trip to London.
The anti-apartheid icon was once again suffering from pneumonia, after he was discharged in April for the same condition, said Maharaj.
"His condition deteriorated to the point where it was found necessary to hospitalise him," he said.
"But I am told by the doctors that he is breathing on his own, so I think that is a positive side," he added.
The announcement, calling his condition "serious", sparked a buzz on Twitter.
"Stay strong mr Mandela heaven isn't ready for you quite yet!" was one plea.
But there were also blunt calls to accept Mandela's increasing frailty and mortality.
"Gosh, Madiba gets no rest from the media. Let him die with dignity. It's not a circus folks," tweeted @Eish_Mag.
In his rural childhood village of Qunu, where Mandela once tended cattle and sheep, the news came via radio for Malunga Mbokodi.
"I was shocked to hear from the radio this morning that he was again in hospital," the 62-year-old told the Sapa news agency.
"I think we should just accept it that Mandela is old and he will go soon," he added.
The ruling ANC called for people at home and abroad to keep "our beloved statesman and icon, Madiba" in their thoughts and prayers.
-- 'Getting too frequent and concerning' --
The Nobel peace laureate is revered as a symbol of forgiveness after leading South Africa into multi-race democracy as its first black president after decades of apartheid rule.
"My thoughts are with Nelson Mandela, who is in hospital in South Africa," tweeted British Prime Minister David Cameron.
The White House also sent good wishes.
"Our thoughts and prayers are with him, his family and the people of South Africa as he recovers," US National Security Council spokeswoman Caitlin Hayden said.
"Getting too frequent and concerning -- he is 94," tweeted Zwelinzima Vavi, general secretary of Cosatu, the powerful labour union allied to the ruling ANC.
Mandela was receiving care at his Johannesburg home when his lung problems returned. He was admitted to an undisclosed hospital after his condition worsened at 01.30am (2330 GMT Friday).
He was diagnosed with early-stage tuberculosis in 1988 and also has had treatment for prostate cancer and suffered stomach ailments.
A heart hospital in Pretoria, where he is believed to have been previously treated, attracted a media camp on Saturday.
An ambulance accompanied by three vehicles with blue lights and sirens sparked excitement after exiting the grounds in the late afternoon, arriving at a military hospital according to local media.
No details were available.
In December, Mandela spent 18 days in hospital, his longest stint since walking free from 27 years in jail in 1990.
In March he was admitted for an overnight scheduled check-up before returning that month for 10-days.
President Jacob Zuma's office said Mandela was "receiving expert medical care and doctors are doing everything possible to make him better and comfortable".
Still a powerful symbol of peace and unity, Mandela has not been seen in public since the World Cup final in July 2010.
After serving just one term he turned his energy to AIDS and conflict resolution, before announcing in 2004 at the age of 85 that he was stepping out of the public eye.
In March, Zuma appeared to prepare the nation for the passing of the father of the "Rainbow Nation".
"In Zulu, when someone passes away who is very old, people say he or she has gone home. I think those are some of the things we should be thinking about," Zuma told the BBC.
Maharaj, who served time with Mandela on apartheid's Robben Island jail, on Saturday said it was good to worry about Mandela but there was no need "to get overworked".
"Whereas in the beginning we would get extremely anxious, this time our anxiety is tempered with a certain understanding of his age and his frailty. It's a good healthy balance," he said.
Television footage in April showed a frail, distant and unsmiling Mandela being visited at home by ANC leaders, sparking accusations that his party was exploiting him.
The African National Congress -- facing elections next year -- has lost much of its Mandela shine, amid widespread corruption, poverty and poor public services.
His own family has also been locked in a feud over control of various companies.