Former South African president Nelson Mandela has been taken to hospital, where he is in a "serious but stable condition", a government spokesman said.
Mr Mandela, 94, was admitted to a hospital in Pretoria at about 1.30am on Saturday after a recurrence of a lung condition.
Mac Maharaj, a spokesman for President Jacob Zuma's office, said: "During the past few days former president Nelson Mandela has had a recurrence of lung infection.
"This morning at about 1.30am his condition deteriorated and he was transferred to a Pretoria hospital. He remains in a serious but stable condition. The former president is receiving expert medical care and doctors are doing everything possible to make him better and comfortable.
"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."
The anti-apartheid leader became president in South Africa's all-race elections in 1994. His health has been failing in recent years.
Mr Maharaj said that members of Mr Mandela's family were with him and had accompanied him to hospital.
He told the BBC that the former president first developed his lung condition many years ago when he was incarcerated in prison, and that he needs treatment from time to time. Doctors treat the matter with great caution, he said, and Mr Mandela is admitted to hospital "as necessary" to give him the best opportunity to recover.
Mr Maharaj added: "He has been in good spirits. He is a fighter and he will be with us as long as he is there fighting."
Prime Minister David Cameron gave his support to Mr Mandela on Twitter. He wrote: "My thoughts are with Nelson Mandela, who is in hospital in South Africa."