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 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 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.
The BBC's Andrew Harding in Johannesburg says the release of information relating to Mr Mandela is always carefully controlled by the government to avoid sparking alarm.
The use of the word "critical" to describe his condition will be seen as very worrying by South Africans, many of whom see him as like a family member, our correspondent says.
There has been little information about his condition in recent days. On 13 June Mr Zuma, who releases all official updates relating to the much-loved leader, said his health continued to improve but that his condition remained serious.
More recently, one of Mr Mandela's grandsons, Ndaba Mandela, said his grandfather was getting better and he hoped he would be home soon.
Mr Mandela's wife, Graca Machel, thanked all those who had sent messages of support, saying they had brought "love, comfort and hope".
Mr Mandela is revered for leading the fight against white minority rule in South Africa and then preaching reconciliation despite being imprisoned for 27 years. He left power after five years as president.
The former president and Nobel Peace Prize winner is believed to have suffered damage to his lungs while working in a prison quarry.
He contracted tuberculosis in the 1980s while being held in jail on the windswept Robben Island.
Mr Mandela retired from public life in 2004 and has rarely been seen at official events since.
On Saturday, it emerged that the ambulance in which Mr Mandela was taken to hospital on 8 June broke down, meaning he had to be moved to another vehicle.
But Mr Zuma said he had been assured that "all care was taken to ensure his medical condition was not compromised".
"There were seven doctors in the convoy who were in full control of the situation throughout the period. He had expert medical care," he said.
Mr Zuma also denied reports that the former leader had suffered a cardiac arrest.