Nelson Mandela has been admitted to hospital with a lung infection.
The former South African president is in a “serious but stable condition”, a spokesman for the current President, Jacob Zuma, says.
Mr Mandela, 94, has been ill for some days but deteriorated overnight and was transferred to a hospital in Pretoria.
Mr Mandela led the fight against apartheid, became South Africa’s first black president, and is widely regarded as father of the nation.
He has recently suffered a series of health problems and this is his fifth visit to hospital in two years.
In April he was released from hospital after a 10-day stay caused by pneumonia.
His illness was described on Saturday as a recurrence of a lung infection, which has troubled him repeatedly.
Mr Mandela was taken to hospital, from his home in a suburb of Johannesburg, at about 01:30 local time (23:30 GMT Friday).
Mac Maharaj, South Africa’s presidential spokesman, told the BBC he was receiving expert medical care.
Doctors were doing everything possible to make him comfortable and better, he added.