A presidential spokesman said he is in a “serious but stable condition”, although he was able to breathe on his own – a “positive sign”.
Mr Mandela, 94, has been ill for some days but deteriorated overnight and was transferred to a hospital in Pretoria.
He led the fight against apartheid and is regarded as the father of democratic South Africa.
He has recently suffered a series of health problems and this is his fifth visit to hospital in two years.
In April he was released from hospital after a 10-day stay caused by pneumonia.
His illness was described on Saturday as a recurrence of a lung infection, which has troubled him repeatedly.
