The government said Mandela needed specialist medical treatment, however the ruling African National Congress (ANC) said his admission was not an emergency and did not involve surgery.
"There's no need for panic," ANC spokesperson Keith Khoza told South Africa's e-News channel. "It was not an emergency admission. It was planned."
An ANC source told Reuters that Mandela, who is popularly known by his clan name Madiba, was "not looking serious".
As South Africa's first black president, Mandela occupies a central position in the psyche of a country that was ruled by the 10 percent white minority until the first all-race elections in 1994. Earlier this month, President Jacob Zuma and the central bank issued a new set of bank notes bearing his image.
Mandela has long since withdrawn from active participation in politics and public life in Africa's biggest economy, having stood down at the end of his first term in office in 1999. His last major public appearance was in July 2010 at the final of the World Cup in Johannesburg's Soccer City stadium.