President Jacob Zuma, accompanied by African National Congress Deputy President Cyril Ramaphosa visited Mandela in his hospital room, SABC reported.
"The condition of former President Nelson Mandela, who is still in hospital in Pretoria, has become critical," a statement issued by Zuma's office afterward said.
Zuma and Ramaphosa were briefed by Mandela's medical team, who informed them the former president's condition had gotten worse in the past 24 hours. They also met with Mandela's wife, Graca Machel and discussed his condition, the network said.
"The doctors are doing everything possible to get his condition to improve and are ensuring that Madiba [Mandela] is well-looked after and is comfortable. He is in good hands," Zuma said.
Mandela, 94, has been in frail health in recent years and has been hospitalized in an intensive care unit since June 8 for a lung infection.
SABC said Zuma and Ramaphosa were assured by the doctors that Mandela's condition was not compromised when the ambulance transporting the former president to the hospital June 8 developed engine problems.
"There were seven doctors in the convoy who were in full control of the situation throughout the period. He had expert medical care," Zuma said. "The fully equipped military ICU ambulance had a full complement of specialist medical staff including intensive care specialists and ICU nurses. The doctors also dismissed the media reports that Madiba suffered cardiac arrest. There is no truth at all in that report."
Zuma asked South Africans and the world to pray for Mandela, and for his family and doctors.