South Africa's first black president, one of the towering figures of modern history, was admitted late on Wednesday for his third spell in hospital in four months.

Doctors drained a build-up of fluid, known as a pleural effusion or "water on the lungs", that had developed from the lung infection.

That procedure helped the former president breathe "without difficulty", Zuma's office said on Saturday.

It was unclear how long Mandela would remain in hospital. Last year he had an 18-day hospital stint in December.

Mandela's recent health troubles saw an outpouring of prayers as Christians celebrated Easter Sunday, but have also seen South Africans come to terms with the mortality of the revered Nobel Peace laureate.

"Yes, we are concerned that he is ailing, and he is getting worse, naturally we should be concerned," Father Sebastian Rossouw told worshippers at the Regina Mundi Catholic Church in Soweto township, once a pivotal centre in the struggle against apartheid.

"I think this is the question in the back of many of our minds, when is the end? Is he going to die? We should rather celebrate what he stood for, and what he continues to stand for, that he has been an icon of peace, an icon of service."

It is the second time that Mandela has been treated in hospital in weeks, after spending a night for check-ups on March 9. That followed a nearly three-week hospital stay in December for another lung infection and gallstone surgery, his longest since he walked free from jail in 1990.

He was diagnosed with early-stage tuberculosis in 1988 during his 27-year jail term and has long had problems with his lungs. He has also had treatment for prostate cancer and has suffered stomach ailments.