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 Pretoria hospital two weeks ago had engine trouble, requiring the former president to be transferred to another ambulance for his journey. Pretoria, South Africa's capital, lies about 50 km (30 miles) from Johannesburg, where Mandela has been living.
"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, a Nobel Peace Prize laureate, 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."
In Washington, the White House National Security Council spokeswoman noted the latest reports from the South African government about Mandela's worsening condition.
"Our thoughts and prayers are with him, his family and the people of South Africa," said spokeswoman Caitllin Hayden.
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. In the days following his latest hospitalization, Zuma's office described his condition as serious but stable. Family members have been seen making daily visits to the hospital where Mandela is being treated.
Mandela, who has become increasingly frail in recent years, last made a public appearance at the 2010 World Cup soccer tournament, which was hosted by South Africa. He didn't deliver an address on that occasion and was bundled against the cold in a stadium full of fans.
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.
Between hospital stays in recent months, Mandela has been staying at his home in the Johannesburg neighborhood of Houghton, where he has received what the government described as "home-based high care" by a medical team. On April 6, he was discharged from a hospital after treatment for pneumonia, which included a procedure in which doctors drained fluid from his lung area.
Mandela has been vulnerable to respiratory problems since contracting tuberculosis during his imprisonment under apartheid. Most of those years were spent on Robben Island, a forbidding outpost off the coast of Cape Town.