JOHANNESBURG (AFP) - 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, who became the first black leader of the country after historic all-race elections in 1994, 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".

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.