Nelson Mandela, the former South African president, was in a "serious but stable" condition after being hospitalised for treatment of a recurring lung infection on Saturday, the president's office said.
It is the fifth time the anti-apartheid leader has been admitted to hospital in two years. He was discharged from hospital in Pretoria in April after 10 days' treatment for pneumonia.
"During the past few days, former President Nelson Mandela has had a recurrence of lung infection," said Mac Maharaj, spokesman for the South African President, Jacob Zuma.
"This morning at about 1.30am, his condition deteriorated and he was transferred to a Pretoria hospital."
Using Mandela's clan name, Maharaj added: "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."
Doctors were doing everything possible to make him comfortable, Maharaj added, while at least one close member of Mandela's family was at his bedside.
"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.
Mandela's wife Graca Machel cancelled an appearance at the Nutrition for Growth hunger summit in London to fly back to South Africa to be with her husband, said the summit's organisers.
Regarded as the "father of the nation", Mandela presided over the period of "truth and reconciliation" which helped unite the country during the dismantling of the apartheid state.
The Nobel Peace Prize laureate was imprisoned for 27 years, and contracted tuberculosis in the 1980s while working in a prison quarry on Robben Island.
On his release, Mandela became the country's first democratically elected president, serving from 1994 to 1999. He retired from public life in 2004 and has since made few public appearances.