President Jacob Zuma visited Mandela at the hospital in Pretoria Sunday, and the medical team informed him that Mandela's condition had become critical since Saturday, Xinhua quoted Zuma's spokesperson Mac Maharaj as saying.
"The doctors are doing everything possible to get his condition to improve and are ensuring that he is well looked after and is comfortable," Zuma said.
"He is in good hands," Zuma added, and appealed to South Africans and the world to pray for Mandela.
Mandela was admitted to hospital in Pretoria with a serious lung infection June 8. On March 27, he was hospitalised after doctors diagnosed him with pneumonia. He was also admitted in December for his recurring lung infection.
Mandela suffered from tuberculosis when he was in prison for 27 years before the apartheid ended in 1994. He was the first democratically-elected president in South Africa.