Anti-apartheid hero Nelson Mandela has slipped into "critical" condition after 16 days in hospital for a recurring lung infection, officials say.
Mr. Mandela, who became the first democratic president of South Africa in 1994 after decades of apartheid, has fallen into critical condition over the past 24 hours but is "comfortable" and "in good hands," President Jacob Zuma announced in a statement on Sunday night.
In previous statements, the 94-year-old former president was always officially described as being in "serious but stable" condition. Last week, the government said his heath was improving.
Mr. Zuma, who visited Mr. Mandela in hospital in Pretoria on Sunday night, said he "appealed to the nation and the world to pray for Madiba, the family and the medical team that is attending to him during this difficult time." He was referring to Mr. Mandela by his affectionate clan name.
"The doctors are doing everything possible to get his condition to improve and are ensuring that Madiba is well looked-after and is comfortable," Mr. Zuma said in the statement. "He is in good hands."
Mr. Zuma was accompanied by Cyril Ramaphosa, deputy president of the ruling African National Congress, in the visit to Mr. Mandela in hospital. "They were briefed by the medical team who informed them that the former president's condition had become critical over the past 24 hours," Mr. Zuma's office said.
A statement by the ANC on Sunday night said it expressed "concern" at the deterioration in Mr. Mandela's condition. "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," it said.
A report by CBS News this weekend said Mr. Mandela had been "unresponsive for days" and was not opening his eyes. Family members were discussing whether to withdraw treatment, the report said.
The government has provided very few details of Mr. Mandela's health, provoking a debate in South Africa over whether more information should be provided to the public.
Some of Mr. Mandela's friends have said publicly that Mr. Mandela's family should "let him go."
The former president, who left office in 1999, has been in hospital four times this year with recurrences of the lung problems that have plagued him for years.
U.S. President Barack Obama is due to arrive in South Africa this Friday as part of a visit to three African countries. White House officials, speaking to journalists on Friday, said they do not know whether Mr. Obama will try to meet Mr. Mandela. They said they will be "very deferential" to the Mandela family on the question of whether to meet him.