Prayers and messages of concern for the ailing 94-year-old, one of the towering figures of modern history, have poured in from across the globe since he was admitted to hospital late Wednesday.
"Mandela is in good spirits and enjoyed a full breakfast this morning," President Jacob Zuma's office said in a statement. "The doctors report that he is making steady progress. He remains under treatment and observation in hospital."
Zuma sought on Thursday to reassure South Africans that the country's anti-apartheid hero and first black president was in good hands.
"The country must not panic, Madiba is fine," Zuma told the BBC, referring to Mandela by his clan name.
"The doctors advise that former president Nelson Mandela is responding positively to the treatment he is undergoing for a recurring lung infection," Zuma's office had said in a short statement Thursday.
It is the second time this month that the Nobel Peace Prize winner has been admitted to hospital, after spending a night for checkups on March 9.
That followed a nearly three-week hospital stay in December, when Mandela was treated for another lung infection and underwent gallstone surgery, after which he was released for home-based care.
Mandela was diagnosed with early-stage tuberculosis in 1988 during his near three-decades in prison under the white-minority apartheid regime and has long had problems with his lungs. He has also had treatment for prostate cancer and suffered stomach ailments.
The latest series of hospitalisations has triggered an outpouring of prayers for Mandela, but has also seen South Africans come to terms with the mortality of their national hero.
"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 said Thursday.
Mandela is idolised in his home nation, where he is seen as the architect of the country's peaceful transition from a white-minority ruled police state to hope-filled democracy.
Nearly 20 years after he came to power in 1994 he remains a unifying symbol in a country still riven by racial tensions and deep inequality.
Labour unrest, high-profile crimes, grinding poverty and corruption scandals have effectively ended the honeymoon enjoyed after Mandela ushered in the "Rainbow Nation".
A figure from another era
Ajith Deena, a visitor to Cape Town, said Mandela is so beloved because he forgave his apartheid captors and said "let's go forward together, let's forget the past and lets move forward as one nation, one country".
He will leave South Africa on "a good footing", Deena told AFP. "It will be a big loss to the country even though he's not in the public eye. It's through him that we are where we are."
While Mandela the symbol bestrides South African politics, the man has long since exited the political stage and for the country's large young population he is a figure from another era, serving as president for just one term from 1994 to 1999.
He has not appeared in public since South Africa's football World Cup final in 2010, six years after retiring from public life.
Still, his nearly life-long struggle against apartheid resonates.
"We are deeply concerned with Nelson Mandela's health -- he is a hero, I think, to all of us," US President Barack Obama said.
"When we think of a single individual that embodies the kind of leadership qualities that I think we all aspire to, the person's name that comes up is Nelson Mandela. So we wish him all the very best," Obama added.
The name and location of the hospital where Mandela is being treated were not disclosed, to allow the medical team to focus on their work and to shield the family from the intense media interest. In the past he has been hospitalised at a clinic in Pretoria.
"We know they are going through a difficult time and we want to ensure that their privacy is maintained," said presidency spokesman Mac Maharaj who served time with Mandela on Robben island.
Away from the public eye Mandela has grown increasingly frail.
His December hospital stay was his longest since he walked free from 27 years of apartheid jail in 1990.
Earlier this month, his friend and renowned human rights lawyer George Bizos, who defended Mandela during his 1960s treason trial, said the ex-president was aware of current political events but was having some memory lapses.
Respiratory infections are often a key cause of death among the elderly, according to pulmonologist Bertrand Dautzenberg.