Nelson Mandela has become critically ill in hospital, the South African presidency has announced.
President Jacob Zuma said he had visited Mr Mandela and spoken to his wife and medical teams.
Doctors were "doing everything possible to get his condition to improve" said Mr Zuma in a statement.
South Africa's first black president, 94, was taken to hospital in Pretoria earlier this month for the third time this year, with a lung infection.
Mr Zuma said he had been told by doctors on Sunday that the former president's condition had worsened over the past 24 hours.
"The doctors are doing everything possible to get his condition to improve and are ensuring that Madiba is well-looked after and is comfortable. He is in good hands," said President Zuma, using Mr Mandela's clan name by which he is widely known in South Africa.
Mr Zuma - who was joined at the hospital by the deputy leader of his ruling African National Congress (ANC) Cyril Ramaphosa - appealed for prayers for Mr Mandela and his medical team.
Mac Maharaj, Mr Zuma's spokesman, told the BBC's Newshour that the doctors' use of the word "critical" was "sufficient explanation that should raise concern amongst us".
"Therefore we want to assure the public that the doctors are working away to try and get his condition to improve," he said.
Mr Maharaj added that this was a stressful time for the Mandela family, and appealed for their privacy.
"I think there is need to be sombre about the news. There is a need not to hold out false hopes but at the same time let's keep him in our thoughts and let's will him more strength," he said.
The ANC - the party of Mr Mandela and Mr Zuma - said it "noted with concern" the latest reports, and that it joined the president in calling "for us all to keep Madiba, his family and medical team in our thoughts and prayers during this trying time".
The White House issued a statement on Sunday saying: "Our thoughts and prayers are with him [Mr Mandela], his family and the people of South Africa."
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