Nelson Mandela is "responding positively" to treatment after being re-admitted to hospital with a lung infection.
The frail 94-year-old former president of South Africa was hospitalised just before midnight on Wednesday, the presidency said.
In a short statement, President Jacob Zuma's office said on Thursday that he remained under treatment and observation in hospital, which has not been named.
The Nobel peace laureate, regarded as the father of the "rainbow nation", was conscious when admitted, presidency spokesperson Mac Maharaj told AFP.
It is the second time in March that Mr Mandela has spent the night in hospital and follows an 18-day stay in December last year for the lung infection and gallstone surgery, after which he was released for home-based care.
Mr Mandela was diagnosed with early-stage tuberculosis in 1988 during his nearly three decades in prison under the white minority apartheid regime and has long had problems with his lungs.
"Doctors are attending to him, ensuring that he has the best possible expert medical treatment and comfort," Jacob Zuma's office said earlier.
Mr Zuma wished "Madiba", as South Africa's first black president from 1994 to 1999 is affectionately known, a quick recovery and asked for people around the world to pray for him.
However, Mr Zuma urged people not to be alarmed about the sudden admission to hospital.
United States President Barack Obama says his thoughts and prayers are with Mr Mandela.
Mr Mandela is revered at home and abroad as the symbol of the country's peaceful shift to democracy after apartheid.
His "long walk to freedom" in 1990 after 27 years in jail was one of the most potent images of the time.
But as he has grown increasingly frail, he has not appeared in public since South Africa's World Cup soccer final in 2010, six years after retiring from public life.
