Prayers have been said in churches across South Africa for former President Nelson Mandela, who is spending a second day in hospital for treatment for a lung infection.The presidency has so far provided no update on his condition although further details are expected later.His condition on Saturday was described as serious but stable.Mr Mandela, 94, had been ill for some days before being taken to a Pretoria hospital early on Saturday.It is the third time this year he has been admitted to hospital.Hundreds of worshippers attended Mass on Sunday at Soweto's Regina Mundi church, famous for its role in the anti-apartheid campaign."I think it's just a natural experience that everybody wants to hold on to him as much as possible," acting priest Father Sebastian Rossouw said."He's always in our prayers."The BBC's Karen Allen in Pretoria says there is a sense of calm across South Africa, and also a quiet hope that the man who led the fight against apartheid may regain his strength once again.Mr Mandela's wife, Graca Machel, cancelled a scheduled appearance in London to remain at her husband's bedside in hospital in Pretoria.On Saturday, presidency spokesman Mac Maharaj said that although Mr Mandela was again suffering from pneumonia, he was breathing on his own - "a positive sign".The former president's close friend Archbishop Desmond Tutu has led calls for South Africans to pray for him and wish him a speedy recovery.Meanwhile, South African President Jacob Zuma has continued to hold engagements away from the capital.He has indicated that he will visit Mr Mandela if doctors advise him to do so.Nelson Mandela served as president from 1994 to 1999.He was previously imprisoned for 27 years, and is believed to have suffered damaged lungs while working in a prison quarry.He contracted tuberculosis in the 1980s while being held in jail on the windswept Robben Island.He retired from public life in 2004 and has been rarely seen in public since.Mr Mandela was receiving care at his Johannesburg home when his latest lung problems returned.He was admitted to hospital after his condition worsened at 01:30 on Saturday (23:30 GMT Friday).British Prime Minister David Cameron has said Mandela is in his thoughts, while the White House has also sent good wishes.