Nelson Mandela is responding positively to hospital treatment for a recurring lung infection, officials say.
The office of South Africa President Jacob Zuma also said that the former president and anti-apartheid leader remains under observation.
Mr Mandela was admitted late on Wednesday to a hospital in Pretoria, the South African capital.
Earlier Mr Zuma said: “We appeal to the people of South Africa and the world to pray for our beloved Madiba and his family and to keep them in their thoughts. We have full confidence in the medical team and know that they will do everything possible to ensure recovery.”
Mr Mandela contracted tuberculosis during his 27-year imprisonment for fighting white racist rule in South Africa. He has repeatedly had lung problems.
He has been admitted to hospital several times since last year, most recently earlier this month when he received what a presidential spokesman described as a “successful” medical test.
Mr Mandela spent a night in a hospital and was released on March 10 after a medical test. At that time, presidential spokesman Mac Maharaj said he was “well”.
In December, Mr Mandela spent three weeks in hospital as he was treated for a lung infection and had a procedure to remove gallstones. A year ago, he was admitted to a Johannesburg hospital for what officials initially described as tests but what turned out to be an abdominal complaint. He was discharged days later.
He had surgery for an enlarged prostate gland in 1985.
Earlier yesterday Presidential spokesman Mac Maharaj acknowledged there was cause for worry, but said the medical specialists treating Mr Mandela were very competent.
“The health has been OK given his age, but the downturn last night – obviously when the lung infection recurs, the doctors will want to do everything possible and make sure that they don’t allow the infection to spread, that they arrest it as quickly as possible,” Mr Maharaj told eNCA, a South African news channel.