The Nelson Mandela Foundation has confirmed that the former president is at Milpark hospital in Johannesbourg undergoing routine tests.
Former South African President Nelson Mandela is recovering from a collapsed lung and could be released from hospital as early as Friday.
The 92-year-old anti-apartheid icon was admitted to a Johannesburg hospital on Wednesday for what his foundation described as routine tests.
The Nelson Mandela Foundation declined to comment on his health on Thursday but said in an earlier statement: "He is in no danger and is in good spirits."
There has been no official word from the hospital, government or Mandela's foundation on the nature of his illness.
President Jacob Zuma and the ruling African National Congress appealed for calm today after the hospitalisation set off speculation in local media about Mandela's health.
President Zuma has said 'President Mandela is comfortable and is well looked after by a good team of medical specialists,' Zuma said in a statement.
ANC spokesman Jackson Mthembu has said' he is a 92-year-old and will have ailments associated with his age, and the fact that he stayed the night should not suggest the worst.'
Several members of Mandela's family, including his wife Graca Machel, visited the hospital after his admission.
Police were called in to control traffic at the Milpark Hospital in a leafy Johannesburg suburb as scores of journalists, photographers and television crews converged on it.
Mandela has not been seen in public since the soccer World Cup final in July last year.
Mandela retired from public life in June 2004 before his 86th birthday, telling his compatriots: 'Don't call me, I'll call you.'
Since then he has rarely appeared in public and when he did, he appeared increasingly frail. In addition to the World Cup, Mandela appeared at a couple of ANC rallies before general elections in 2009.
South African Nobel Peace Prize laureate Desmond Tutu said this week he had met Mandela last week. 'He was alright, I mean, he's 92, you know. And he's frail.'
