There was no official update on 94-year-old Mandela's condition, described in a government statement on Saturday as "serious but stable."
The office of President Jacob Zuma had said that Mandela was taken to a Pretoria hospital after his condition deteriorated at around 1:30 a.m. on Saturday. It described the lung infection as recurring.
The anti-apartheid leader has now been taken to a hospital four times since December, with the last discharge coming on April 6 after doctors diagnosed him with pneumonia and drained fluid from his lung area.
Worshippers at a Sunday church service in the Johannesburg township of Soweto prayed for the recovery of Mandela, who was freed in 1990 after 27 years as a prisoner of white racist rule and won election to the presidency in all-race elections in 1994.
At the Regina Mundi church in Soweto, Father Sebastian Rousso said Mandela, seen by many as a symbol of reconciliation, played a key role "not only for ourselves as South Africans, but for the world."
Welcome Tempa, a construction worker, said he prayed daily for Mandela, who retired from public life years ago and had been receiving medical care at his Johannesburg home until his latest transfer to a hospital.
"When I was still a kid, we used to pray for him," said Tempa, who is 63 years old.
On April 29, state television broadcast footage of a visit by Zuma and other leaders of the ruling 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.
The Nobel Peace Prize laureate has been particularly vulnerable to respiratory problems since contracting tuberculosis during his long imprisonment. The bulk of that period was spent on Robben Island, an outpost off the coast of Cape Town where Mandela and other prisoners spent part of the time toiling in a stone quarry.