The government said Mandela needed specialist medical treatment although 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 spokesman 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".
Mandela has been in poor health since spending nearly a week in Johannesburg's Milpark hospital just over a year ago with respiratory problems, and has not appeared in public since then.
He has spent the last year between his home in Johannesburg's northern suburbs and his ancestral village of Qunu in the impoverished Eastern Cape.
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Mandela has long since withdrawn from active participation in politics and public life in Africa's biggest economy, having stood down at the end of his first term in office in 1999.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.
(Reporting by Ed Cropley and Peroshni Govender; Editing by Janet Lawrence/Ruth
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