President Jacob Zuma said in a statement that the anti-apartheid icon remains in “serious but stable” condition while being treated in intensive care.
“Stable has not meant better or worse, what it means is that his condition is not changing,” Zuma’s spokesman Mac Maharaj told AFP.
Maharaj declined to give further details about the state of the much-loved icon, citing issues of “patient-doctor confidentiality”.
Amid growing fears about the 94 year-old’s condition, family members were seen entering the Mediclinic Heart Hospital in Pretoria to be at his bedside.
Security was tightened around the private specialist facility where a dozen armed police stood guard outside and incoming vehicles and pedestrians were searched amid a heavy media presence.
“They are there to protect the members of his family who come to visit him,” a police sergeant told AFP, asking not to be named.
His daughters Makaziwe and Zindzi, as well as former wife Winnie Madikizela-Mandela, were seen entering the hospital on Tuesday afternoon.
Mandela’s current wife Graca Machel called off a trip to London last week to be with her ailing husband.
Zuma’s office said he had a “thorough briefing” from Mandela’s doctors late Monday.
“President Zuma has full confidence in the medical team, and is satisfied that they are doing their best to make Madiba better,” said the statement, using Mandela’s clan name.
Medical experts say the recurrent infection could be life threatening.
Tuesday marked 49 years to the day since Mandela was sentenced to life in prison in 1964 for conspiring to overthrow the apartheid government.
He spent much of the subsequent 27 years behind bars on wind-swept Robben Island, near Cape Town, where he contracted tuberculosis.
Mandela’s latest health scare has been met with a growing acceptance among South Africans that their hero, who became the first black leader of the country after historic all-race elections in 1994, may be nearing the end of his life.
He has a long history of lung problems since being diagnosed with early-stage tuberculosis in 1988, and this is the fourth hospital stay since December for the Nobel Peace Prize winner and father of the “Rainbow Nation”.
In December he underwent surgery to remove gallstones as he recovered from a lung infection. Then in March he was admitted for a scheduled overnight check-up before returning later that month for 10 days.
“Pneumonia is a killer disease,” said Keertan Dheda, the head of pulmonology at the University of Cape Town.
“In Mr Mandela’s case, besides age, we know that he has previously had tuberculosis and that can weaken the lung defences and make one more prone to infections,” he said.
“Secondly we know that Mandela worked in the quarry on Robben Island for many years and he has been chronically exposed to dust from crushing rocks, and we know that also is a potent suppressor of your efficient lung defences.”
Access to the revered statesman has been restricted to close family members.
In late April, Zuma and top party officials were photographed with an unsmiling Mandela looking exceedingly frail at his Johannesburg home.
The visit prompted allegations that the embattled ruling party was exploiting Mandela for political gain.
The ANC, facing 2014 elections, has lost much of its Mandela shine amid widespread corruption, poverty and poor public services.
Mandela has not been seen in public since the World Cup final in South Africa in July 2010, and has not been politically active for years.
“I think there will be concerns from outside South Africa that Mandela is seen as the glue that holds South Africa together,” analyst Daniel Silke told AFP.
“But I think that this is something long gone, frankly.”
After serving just one term as president, Mandela turned his energy to the battle against AIDS and to conflict resolution, before stepping out of the public eye at age 85.
His hospitalisation has prompted an outpouring of well wishes from around the globe, including from fellow Nobel peace laureate Archbishop Desmond Tutu.
“As the beloved father of our nation… once again endures the ravages comfort and his dignity,” Tutu said in a statement from his foundation
Ordinary people, young and old, on Tuesday left messages of support outside his home in northern Johannesburg.
A couple wearing t-shirts bearing the words “We love you Papa Mandela” and “We love you Papa Madiba” placed a teddy bear in a similar t-shirt at one of the pot plants outside the gate.
Others wrote messages of support on small stones outside the high security walls, while a group of schoolchildren stopped by to sing “get well Tata Mande, get well”.