"We hope that it is true — as reported by the presidential spokesperson — there is no cause for concern or alarm," said spokesperson Patrick Craven in a statement.
Mandela (94) was admitted to a Pretoria hospital for medical tests, just short of a year since his last known hospital visit.
On Sunday morning, worshipers gathered at the Regina Mundi Catholic church in Soweto to pray for the frail icon. The church was a center of anti-apartheid protests and funerals.
"President Jacob Zuma wishes to advise that former president Nelson Mandela has today, 8 December 2012, been admitted in hospital in Pretoria to undergo tests," said spokesperson Mac Maharaj in a statement on Saturday afternoon.
The hospital was not named.
Craven said Madiba had always been a constant inspiration, through apartheid, jail and exile, the mass uprisings of the Eighties, negotiations, and the days of liberation and reconstruction.
"His leadership was key to our ultimate breakthrough," Craven said.
"Get well and continue to inspire us."
Maharaj insisted that there was no cause for concern.
"As said before, former president Mandela will receive medical attention from time to time which is consistent with his age."
Mandela was admitted to hospital in February this year for a diagnostic procedure, which Lindiwe Sisulu, defence minister at the time, later said was "an investigative laparoscopy".
"President Zuma assures all that Madiba is doing well and there is no cause for alarm," Maharaj said.
Zuma said: "We wish Madiba all the best. The medical team is assured of our support as they look after and ensure the comfort of our beloved founding president of a free and democratic South Africa."
The presidency appealed to the media and the public to respect the privacy of Mandela and his family. – Sapa