Nelson Mandela hospitalized
Current South African President Jacob Zuma has said in a statement: "We appeal to the people of South Africa and the world to pray for our beloved Madiba [a nickname for Mandela] and his family and to keep them in their thoughts."
"We have full confidence in the medical team and know that they will do everything possible to ensure recovery," he added. "The Presidency appeals once again for understanding and privacy in order to allow space to the doctors to do their work."
Mandela spent nearly three weeks in a hospital in December for treatment of a lung infection and gallstone surgery.
This was his longest time he had been hospitalized since being released from captivity as a political prisoner in 1990.
Mandela was president of South Africa from 1994 to 1999, the first president of the country to be elected following the fall of the apartheid system.
NBC News contributed to this report.