JOHANNESBURG - South African police on Tuesday tightened security at the Pretoria hospital where ailing anti-apartheid icon Nelson Mandela was spending his fourth day in intensive care battling a lung infection.

Around a dozen armed police stood guard outside the Mediclinic Heart Hospital in the capital, where the 94-year-old remained in “serious but stable” condition. Mandela, one of the greatest figures of the 20th century, is being treated for a recurrent lung infection that medical experts say could be life threatening. Tuesday marked 49 years to the day since he was sentenced to life in prison in 1964 for conspiring to overthrow the apartheid government.

Mandela spent much of the subsequent 27 years behind bars on wind-swept Robben Island, near Cape Town, where he contracted tuberculosis.

His latest health scare has been met with prayers and a growing acceptance among South Africans that their hero may be nearing the end of his life.

In Pretoria, police cordoned off an area in front of the private specialist facility, searching incoming vehicles and pedestrians amid a heavy media presence.

“They are there to protect the members of his family who come to visit him,” a police sergeant told AFP, asking not to be named because Mandela’s location has not been confirmed by the government.

On Monday, the arrival of his former wife Winnie Madikizela-Mandela and the couple’s daughter Zindzi prompted a scuffle between security and photographers camped outside the clinic.

Madikizela-Mandela became a global political figure in her own right while campaigning for her husband’s release from prison.

His current wife Grace Machel called off a trip to London last week to be with her ailing husband.

Little information has been released about Mandela’s condition, but he has a long history of lung problems since being diagnosed with early-stage tuberculosis in 1988.

It is the fourth hospital stay since December for the Nobel Peace Prize winner and father of the “Rainbow Nation”.

Two months ago Mandela, who turns 95 next month, was discharged following treatment for pneumonia.

In December he underwent surgery to remove gallstones as he recovered from a lung infection. In March he was admitted for a scheduled overnight check-up before returning later that month for 10 days.

“Pneumonia is a killer disease,” said Professor Keertan Dheda, the head of pulmonology at the University of Cape Town.

“In Mr Mandela’s case, besides age, we know that he has previously had tuberculosis and that can weaken the lung defences and make one more prone to infections,” he said.

“Secondly we know that Mandela worked in the quarry on Robben Island for many years and he has been chronically exposed to dust, from crushing rocks and we know that also is a potent suppressor of your efficient lung defences.”

But Dheda said assurances that Mandela, once a spry boxer, was breathing on his own indicated that the infection had not yet led to serious lung failure.

Access to the revered statesman has been restricted to close family members.

In late April, President Jacob Zuma and top party officials were photographed with an unsmiling Mandela looking exceedingly frail at his Johannesburg home.

The visit prompted allegations that the embattled ruling party was exploiting Mandela for political gain.

The ANC - facing 2014 elections - has lost much of its Mandela shine amid widespread corruption, poverty and poor public services.

Mandela has not been seen in public since the World Cup final in South Africa in July 2010, and has not been politically active for years.

“I think there will be concerns from outside South Africa that Mandela is seen as the glue that holds South Africa together,” analyst Daniel Silke told AFP.

“But I think that this is something long gone frankly.”

After serving just one term as president, Mandela turned his energy to the battle against AIDS and to conflict resolution, before stepping out of the public eye a decade ago at the age of 85.

His hospitalisation has prompted an outpouring of well wishes from around the globe, including from fellow Nobel peace laureate Archbishop Desmond Tutu.

“We offer our thanks to God for the extraordinary gift of Mr Mandela, and wish his family strength,” Tutu said in a statement from his foundation.

“As the beloved father of our nation... once again endures the ravages of time in hospital, our prayers are for his comfort and his dignity.”