The condition of former President Nelson Mandela, who is still in hospital in Pretoria, has become critical," spokesman Mac Maharaj said in a statement on Sunday night.
He said President Jacob Zuma and ANC deputy president Cyril Ramaphosa visited Mandela, who was admitted with a recurring lung infection on June 8, on Sunday evening.
"They were briefed by the medical team who informed them that the former president's condition had become critical over the past 24 hours."
Zuma and Ramaphosa also met with Mandela's wife Graca Machel to discuss Mandela's condition.
"The doctors are doing everything possible to get his condition to improve and are ensuring that Madiba is well-looked after and is comfortable," Zuma said in the statement.
"He is in good hands."
He said the doctors also dismissed reports on Saturday that Mandela had suffered cardiac arrest .
"There is no truth at all in that report,'' said Zuma.
Zuma appealed to the nation and the world to pray for Madiba,
the family and the medical team that is attending to him.
In an interview on ENCA, Maharaj added: "We are not in a position to go into clinical detail because we have to respect the ethical issues of doctor/patient confidentiality, privacy and human dignity."
The information released to the media was based on what doctors gave the presidency and Machel.
Sunday's statement also reiterated that Mandela had expert medical care with him at the time his ambulance broke down on his way to hospital, after reports on this on Saturday.