JOHANNESBURG—South Africa's iconic former leader Nelson Mandela has lapsed into critical condition after spending more than two weeks in a Pretoria hospital to treat a lung infection, according to a statement from the president's office.

President Jacob Zuma and deputy president of the ruling African National Congress, Cyril Ramaphosa, visited Mr. Mandela at the hospital Sunday evening. The medical team treating Mr. Mandela told the pair of visitors the condition of the 94-year-old former president had "become critical...