Nelson Mandela's health has been a cause of concern for some time
Former South African President Nelson Mandela is "responding positively" to treatment for the recurrence of a lung infection, the presidency says.
A statement said the 94-year-old anti-apartheid leader remained under treatment and observation.
Mr Mandela was admitted to hospital just before midnight, the office of President Jacob Zuma said earlier.
He spent 18 days in hospital in December undergoing treatment for a lung infection and gallstones.
The presidency has not identified the hospital where Mr Mandela is being treated.
In an update on his condition, the presidency statement thanked the media and the public "for their co-operation in respecting the privacy of Madiba [Mr Mandela] and his family".
Mr Mandela is regarded by many South Africans as the father of the nation for leading the struggle against apartheid.
He served as South Africa's first black president from 1994 to 1999. However, his health has caused concern for some time.
Earlier, a presidential spokesman told the BBC that Mr Mandela was conscious in hospital.
"I think we need to be clear that the doctors are attending to Madiba [Mr Mandela] on a continuous basis," spokesman Mac Maharaj said.
"They prefer to act on the side of caution, and the moment they felt there was a recurrence of the lung infection, they felt that it warranted immediate hospitalisation given his age and given his history."
President Zuma urged people around the world to pray for the former leader.
"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," President Zuma said in a statement.
The former president is often fondly referred to by his clan name, Madiba.
The governing African National Congress also called for prayers for Mr Mandela.
"During these trying times we wish President Mandela well and for his family to be strong," the ANC said in a statement.
"We are confident that the treatment will be successful as he is in professional and competent hands," it added.
It is the fourth time Mr Mandela has been admitted to hospital in just over two years.
He first contracted tuberculosis in the 1980s while detained on the windswept Robben Island where he served 18 of the 27 years he was imprisoned for sabotage.
His lungs are said to have been damaged when he worked in a prison quarry.
Despite his long imprisonment, Mr Mandela forgave his former enemies and as president urged South Africans of all races to work together and seek reconciliation.
In 1993 he was awarded the Nobel Peace Prize.
The treatment Mr Mandela received in December 2012 was his longest spell in hospital since leaving prison in 1990.
Earlier this month he spent a night in hospital following a check-up.
Mr Mandela retired from public life in 2004 and has been rarely seen in public since.
His main home is in Qunu, a small rural village in Eastern Cape province, where he says he spent the happiest days of his childhood.
However, doctors said in December he should remain at his home in the Johannesburg neighbourhood of Houghton to be close to medical facilities.