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 the country's first black president, and is regarded as the father of democratic South Africa.
He has recently suffered a series of health problems and this is his fifth visit to hospital in two years.
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As long as he is still alive... we still have hope. South Africa is nothing without him”
Mamoshomo Tswai
Trader, Pretoria
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.
Mr Maharaj said at least one close member of Mr Mandela's family was with him in hospital.
"Naturally the immediate members of the family have access to him and it's always good for the patient that he has been accompanied by one or other of them, and that has happened," he said.
Damaged lungs
"President Jacob Zuma, on behalf of government and the nation, wishes Madiba a speedy recovery and requests the media and the public to respect the privacy of Madiba and his family," Mr Maharaj said in a statement, using the clan name by which Mr Mandela is often known.
On the streets of Pretoria, people expressed their affection for their former president and their concern.
Mamoshomo Tswai, a trader, said: "As long as Tata [father] is still alive then poor people like me, people who are down down, single mothers like me, we still have hope. South Africa is nothing without him."
But another informal trader in Pretoria, who did not want give their name, said: "We must just accept that he is old. We love him, we all do, but we must start to accept that he is a very old man."
Mr Mandela served as president from 1994 to 1999.
He was previously imprisoned for 27 years, and is believed to have suffered damaged lungs while working in a prison quarry.
He contracted tuberculosis in the 1980s while being held in jail on the windswept Robben Island.
He retired from public life in 2004 and has been rarely seen in public since.
There was a row in April when South Africa's governing African National Congress (ANC) - Mr Mandela's party - filmed a visit to see him and broadcast the pictures of him with President Zuma and other party figures.
Critics called it an invasion of his privacy.
Mr Mandela awarded the Nobel Peace Prize in 1993 jointly with former President FW de Klerk for ending apartheid and bringing democracy to South Africa.
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 the country's first black president, and is regarded as the father of democratic South Africa.
He has recently suffered a series of health problems and this is his fifth visit to hospital in two years.
Continue reading the main story
“
Start Quote
As long as he is still alive... we still have hope. South Africa is nothing without him”
Mamoshomo Tswai
Trader, Pretoria
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.
Mr Maharaj said at least one close member of Mr Mandela's family was with him in hospital.
"Naturally the immediate members of the family have access to him and it's always good for the patient that he has been accompanied by one or other of them, and that has happened," he said.
Damaged lungs
"President Jacob Zuma, on behalf of government and the nation, wishes Madiba a speedy recovery and requests the media and the public to respect the privacy of Madiba and his family," Mr Maharaj said in a statement, using the clan name by which Mr Mandela is often known.
On the streets of Pretoria, people expressed their affection for their former president and their concern.
Mamoshomo Tswai, a trader, said: "As long as Tata [father] is still alive then poor people like me, people who are down down, single mothers like me, we still have hope. South Africa is nothing without him."
But another informal trader in Pretoria, who did not want give their name, said: "We must just accept that he is old. We love him, we all do, but we must start to accept that he is a very old man."
Mr Mandela served as president from 1994 to 1999.
He was previously imprisoned for 27 years, and is believed to have suffered damaged lungs while working in a prison quarry.
He contracted tuberculosis in the 1980s while being held in jail on the windswept Robben Island.
He retired from public life in 2004 and has been rarely seen in public since.
There was a row in April when South Africa's governing African National Congress (ANC) - Mr Mandela's party - filmed a visit to see him and broadcast the pictures of him with President Zuma and other party figures.
Critics called it an invasion of his privacy.
Mr Mandela awarded the Nobel Peace Prize in 1993 jointly with former President FW de Klerk for ending apartheid and bringing democracy to South Africa.
