The office of President Jacob Zuma said in a statement that he had visited the 94-year-old anti-apartheid leader at a hospital Sunday evening and was informed by the medical team that Mandela's condition had become critical in 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," Zuma said in the statement, using Mandela's clan name.
Zuma also met Graca Machel, Mandela's wife, at the hospital in Pretoria and discussed the former leader's condition, according to the statement. Zuma was accompanied on the visit by Cyril Ramaphosa, the deputy president of the country's ruling party, the African National Congress.
Mandela was jailed for 27 years under white racist rule and released in 1990. He then played a leading role in steering the divided country from the apartheid era to democracy, becoming South Africa's first black president in all-race elections in 1994. He was hospitalized on June 8 for what the government said was a recurring lung infection.
In Sunday's statement, Zuma also discussed the government's acknowledgement a day earlier that an ambulance carrying Mandela to the hospital two weeks ago had engine trouble, requiring the former president to be transferred to another ambulance for his journey to the hospital.
"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."
Mandela is seen by many around the world as a symbol of reconciliation, and Zuma appealed to South Africans and the international community to pray for the ailing ex-president, his family and the medical team attending to him.
The ruling party expressed concern about the deterioration in Mandela's health.
"We welcome the work being done by The Presidency to ensure that South Africans and people of the world are kept informed on the state of Madiba's health," the party said. "The African National Congress joins The Presidency in calling upon all of us to keep President Mandela, his family and his medical team in our thoughts and prayers during this trying time."
Prior to Zuma's statement late Sunday, reports from the government, former President Thabo Mbeki and a grandson of Mandela had indicated that the health of Mandela was improving, even though he has been in the hospital for treatment several times in recent months. Mandela last made a public appearance in 2010, and has become increasingly frail in recent years.
On April 29, state television broadcast footage of a visit by Zuma and other leaders of the African National Congress to Mandela's home. Zuma said at the time that Mandela was in good shape, but the footage - the first public images of Mandela in nearly a year - showed him silent and unresponsive, even when Zuma tried to hold his hand.