Family members of former South African President Nelson Mandela were seen visiting a Pretoria hospital on Sunday where he is reported to be receiving medical treatment for a lung infection for a second day.

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There was no official update on the 94-year-old's condition, described in a government statement on Saturday as "serious but stable."

The office of President Jacob Zuma had said Mandela was taken to a Pretoria hospital early Saturday after his condition deteriorated.

Worshippers at a Sunday church service in the Johannesburg township of Soweto prayed for Mandela's recovery.

At the Regina Mundi church in Soweto, Father Sebastian Rousso said Mandela, seen by many as a symbol of reconciliation, played a key role "not only for ourselves as South Africans, but for the world."

South Africa's The Sunday Times newspaper took a sombre tone, with the headline "It's time to let him go," quoting a longtime friend of Mandela's.

"We wish Madiba (Nelson Mandela) a speedy recovery, but I think what is important is that his family must release him," Andrew Mlangeni said.

"You have been coming to the hospital too many times. Quite clearly you are not well and there is a possibility you might not be well again," Mlangeni said.

The anti-apartheid leader has now been taken to a hospital four times since December, with the last discharge coming on April 6 after doctors diagnosed him with pneumonia and drained fluid from his lung area.

The Nobel Peace Prize laureate served as president from 1994 to 1999, after a long fight against South Africa's apartheid laws.

He was previously imprisoned for 27 years, and contracted tuberculosis while in custody, which damaged his lungs.

The bulk of that period was spent on Robben Island, an outpost off the coast of Cape Town where Mandela and other prisoners spent part of the time toiling in a stone quarry.