"His condition is unchanged," the office of President Jacob Zuma said in a brief statement.
Mandela, who is 94 years old, was taken to a hospital early Saturday to be treated for a recurring lung infection. At that time, Zuma's office described the anti-apartheid leader's condition as "serious but stable."
On Sunday, members of Mandela's family were seen visiting the hospital where the anti-apartheid leader and Nobel Peace Prize laureate is believed to be staying.
Mandela, also known by his clan name Madiba, has been hospitalized several times in recent months. During a hospital stay that ended April 6, doctors diagnosed him with pneumonia and drained fluid from his chest.
"President Jacob Zuma reiterates his call for South Africa to pray for Madiba and the family during this time," the presidential statement said.
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 then 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.
Mandela has been particularly vulnerable to respiratory problems since contracting tuberculosis during 27 years as the prisoner of the white racist government. The bulk of that period was spent on Robben Island, off the coast of Cape Town where Mandela and other prisoners spent part of the time toiling in a stone quarry.
Mandela was freed in 1990 and won election to the presidency in the country's first all-race elections in 1994. He was seen by many around the world as a symbol of resolve and reconciliation for his sacrifice in confinement as well as his peacemaking efforts during the tense transition that saw the demise of the apartheid system.
The former leader retired from public life years ago and had received medical care at his Johannesburg home until his latest transfer to a hospital.