Nelson Mandela's condition has deteriorated to ''critical''.
According to the Presidency, the 94-year-old former president's health had worsened since Saturday night.
President Jacob Zuma, accompanied by ANC deputy president Cyril Ramaphosa, visited Madiba at Medi-Clinic Heart Hospital in Pretoria last night.
In an update at 9.30pm, the Presidency said Zuma and Ramaphosa were briefed by Mandela's medical team, who informed them that Mandela's condition had become "critical over the past 24 hours".
The ANC leaders also met Mandela's wife, Graca Machel, who has been at Madiba's bedside since he was hospitalised more than two weeks ago.
"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," Zuma said last night.
Mandela, who has been in the hospital since June 8, is suffering from a recurring respiratory infection.
His sudden deterioration yesterday followed reports late last week that his condition was improving.
The anti-apartheid icon's history of lung problems dates back to his time on Robben Island.
Mandela has been hospitalised four times with respiratory problems since December .
His continued stay in the hospital has been an anxious time for millions of South Africans.
Zuma and the ANC last night called on the nation to keep Mandela, his family and his medical team in their thoughts and prayers.
The update came a day after it was reported that the ambulance transporting Mandela to hospital in the early hours of June 8 broke down in sub-zero temperatures.
Weekend reports said Madiba waited for about 45 minutes on the side of the road for another military ambulance to be dispatched.
But yesterday, the Presidency said all care had been taken at the time to ensure that Mandela's medical condition was not compromised.
Zuma said last night: "There were seven doctors in the convoy who were in full control of the situation throughout the period" when Mandela was stranded on the roadside.
"He had expert medical care. The fully equipped [intensive care] military ambulance had a full complement of specialist medical staff, including intensive care specialists and nurses," Zuma said.
He said doctors had also dismissed claims that Madiba had suffered cardiac arrest en route to hospital.
The news of Mandela's worsening condition spread across the world last night, with many South Africans posting their thoughts and fears on social media.
Several news crews were seen camping outside the Pretoria hospital after news broke that Mandela had taken a turn for the worse.
After 10pm news crews were off-loading and setting up broadcast equipment, including lights and generators.