The 94-year-old Nobel Peace Prize laureate has received hospital treatment several times in recent months. His health has been failing in recent years.
"During the past few days former President Nelson Mandela has had a recurrence of lung infection. This morning at about 1.30am his condition deteriorated and he was transferred to a Pretoria hospital," said a statement from the office of President Jacob Zuma.
"He remains in a serious but stable condition," it said.
The statement said Mandela was receiving expert medical care and "doctors are doing everything possible to make him better and comfortable. President Zuma, on behalf of government and the nation, wishes Madiba a speedy recovery," it said, requesting the media and the public to respect the privacy of Mandela and his family.
Earlier in March, Mandela was admitted to hospital with a recurring lung infection. Mandela had a long history of lung problems, dating back to the time when he was a political prisoner on Robben Island during apartheid. While in jail he contracted tuberculosis.
Mandela served as South Africa's first black president from 1994 to 1999 and is widely regarded as the father of the nation for leading the struggle against apartheid and for democracy.
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