The 94-year-old was taken to a hospital to be treated for a lung infection. His wife, humanitarian activist Graca Machel, canceled an appearance at an international forum on hunger and nutrition in London on Saturday, citing "personal reasons," said Colleen Harris, a spokeswoman for the meeting.
Mandela has been in and out of the hospital several times in recent months. Most recently he was in the hospital with pneumonia back in April. He has been particularly vulnerable to respiratory problems since contracting tuberculosis during his 27-year imprisonment under apartheid.
According to a statement from the office of President Jacob Zuma, Mandela's condition deteriorated early Saturday morning, and he was transferred to a Pretoria hospital. The statement said Mandela was receiving expert medical care and "doctors are doing everything possible to make him better and comfortable."
Zuma wished Mandela a quick recovery on behalf of the government and the nation and requested that the media and the public respect the privacy of the former leader and his family, the statement said.
Mandela was robust during his decades as a public figure, endowed with charisma, a powerful memory and an extraordinary talent for articulating the aspirations of his people and winning over many of those who opposed him.
In recent years, however, he has become more frail and last made a public appearance at the 2010 World Cup soccer tournament, where he didn't deliver an address and was bundled against the cold.
The anti-apartheid leader became president in South Africa's all-race elections in 1994.
The Associated Press contributed to this report.