President Jacob Zuma and ANC deputy president Cyril Ramaphosa visited Mandela last evening, Mac Maharaj, presidential spokesman, said in a statement.
The condition of 94-year-old who was admitted to hospital on June 8 has worsened in the past 24 hours. "They were briefed by the medical team who informed them that the former president's condition had become critical over the past 24 hours," Maharaj said.
"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," the statement quoted Zuma as saying.
They also discussed his condition with Mandela's wife Graca Machel, who is believed to have been staying in an adjacent room at the hospital.
The latest statement from the Presidency is the strongest indication yet that all is not well with the ailing elder statesman, who will turn 95 in three weeks' time.
The news came amid a mounting public outcry after it was learnt that the ambulance transporting Mandela to hospital from his home in Johannesburg in the early hours of June 8 broke down and paramedics had to treat him for almost forty minutes before a second one arrived.