A statement from President Jacob Zuma's office says Mandela had a recurrence of the illness in the last few days, and that he was transferred to a Pretoria hospital after his condition deteriorated at about 01:30 on Saturday.
The statement says Mandela, who is 94 years old, is in "serious but stable" condition and is receiving expert medical care.
The anti-apartheid leader became president in South Africa's all-race elections in 1994. His health has been failing in recent years.
