South African ex-President Nelson Mandela is spending a fourth day in a Pretoria hospital where he is being treated for a recurrent lung infection.The office of President Jacob Zuma says the 94-year-old remains in a serious but stable condition.Relatives, including his ex-wife Winnie Madikizela-Mandela, visited the ailing former leader on Monday.He has been in intensive care since he was was admitted to hospital on Saturday, for the third time this year.In December, Mr Mandela spent 18 days undergoing treatment for a lung infection and gallstones.PrayersThe foundation led by retired Archbishop Desmond Tutu has issued a statement describing Mr Mandela as an "extraordinary gift"."As the beloved father of our nation, Nelson Mandela once again endures the ravages of time in hospital," the foundation said."Our prayers are for his comfort and his dignity."There are first signs that his family may have begun to gather at his bedside, the BBC's Andrew Harding, in Pretoria, reports.On Monday, Mr Mandela's ex-wife, Winnie Madikizela-Mandela, visited him. His grand-daughters have also been coming and going, our correspondent says.The leader's eldest daughter, Zenani Mandela-Dlamini, has returned to South Africa from Argentina, where she is the ambassador, to visit her father.His wife, Graca Machel, cancelled a scheduled appearance in London on Saturday to remain at her husband's bedside.In an earlier statement, the government said: "President Jacob Zuma reiterates his call for South Africa to pray for Madiba and the family during this time." Madiba is Mr Mandela's clan name.Presidency spokesman Mac Maharaj also denied reports that visitors were being barred from seeing Mr Mandela.He said: "The reality is that the normal procedures when a patient is under intensive care are applying from the medical side."When a person is in intensive care the doctors only allow some very close people to be there."The presidency also said Mr Mandela had been ill for some days at his Johannesburg home, with a recurrence of his long-standing lung problems.He was admitted to hospital after his condition worsened at 01:30 on Saturday (23:30 GMT Friday).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.