Mandela, who is 94 years old, was treated in a hospital several times in recent months, with the last discharge coming on April 6 after doctors diagnosed him with pneumonia and drained fluid from his lung area. He has been particularly vulnerable to respiratory problems since contracting tuberculosis during his 27-year imprisonment under apartheid.
A small girl and her father stood outside Mandela's Johannesburg home with a stone on which was written a get-well message for Mandela, who helped end white racist rule and became the country's first black president in all-race elections in 1994. A young boy brought a bouquet of flowers that he handed over to guards at the house.
Elsewhere in the city, some worshippers prayed for Mandela during an outdoor gathering.
"If the time comes, we wish for him a good way to go," said Noel Ngwenya, a security officer who was in the congregation.
"During the past few days, former President Nelson Mandela has had a recurrence of lung infection," said a statement from the office of President Jacob Zuma. "This morning at about 1:30 a.m., his condition deteriorated and he was transferred to a Pretoria hospital."
It 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's 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.
Presidential spokesman Mac Maharaj said Machel had canceled her attendance at the London meeting on Thursday, and had accompanied Mandela to the hospital on Saturday morning, the South African Press Association reported.
"We need to hold our thoughts and keep him in our minds," Maharaj said. "He is a fighter, he has recovered many times from very serious conditions and he will be with us. Let's pray for him and help him to get better."
The African National Congress, the ruling party that has dominated politics in South Africa since the end of apartheid, said it hoped Mandela, known affectionately by his clan name Madiba, would get better soon.
"We will keep President Mandela and his family in our thoughts and prayers at this time and call upon South Africans and the peoples of the globe to do the same for our beloved statesman and icon, Madiba," the party said in a statement.
On April 29, state television broadcast footage of a visit by Zuma and other ANC leaders to Mandela at his Johannesburg 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.
"Nelson Mandela is a father to South Africa and South Africans; every time he is admitted to hospital we feel saddened along with the rest of our country," the Democratic Alliance, the main political opposition party, said in a statement.
South Africans expressed hope that Mandela would recover from his latest setback.
"He is going to survive," said Willie Mokoena, a gardener in Johannesburg. "He's a strong man."
Another city resident, Martha Mawela, said she thought the former president would recover because: "Everybody loves Mandela."
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.
In another recent hospitalization, Mandela was treated for a lung infection and had a procedure to remove gallstones in December. In March, he spent a night in a hospital for what authorities said was a scheduled medical test.