THE hospital where Nelson Mandela is in intensive care has tightened security after the ailing democracy icon's daughters paid him a visit.

Security was tightened around the private facility, where a dozen armed police stood guard outside and incoming vehicles and pedestrians were searched amid a heavy media presence.

A police sergeant told AFP officers had been deployed at the hospital "to protect the members of his family who come to visit him.''

Mr Mandela's daughters Makaziwe and Zindzi, as well as former wife Winnie Madikizela-Mandela, were seen entering the hospital on Tuesday afternoon.

Mandela's relatives flocked to the Mediclinic Heart Hospital in Pretoria to be at his bedside as fears grew over his condition. His current wife, Graca Machel, called off a trip to London last week to be with her ailing husband.

The increased security came as South Africa's government said Mr Mandela remained in "serious but stable" condition as he receives treatment for a lung infection.

Mr Mandela's doctors briefed South African President Jacob Zuma on the former president's health late Monday, the president said in a statement. In an interview, Mr Zuma called Mr Mandela's situation "very serious'' but said he has stabilised.

"We need him to be with us and I'm sure that all the messages that have been pouring in to wish him (a) quick and speedy recovery, they're highly welcome,'' Mr Zuma told public broadcaster SABC.

"We need him to be with us,'' Mr Zuma said. "And I'm sure, knowing him as I do, he's a good fighter and he'll be with us very soon.''

Zuma's spokesman Mac Maharaj earlier told AFP that "stable has not meant better or worse, what it means is that his condition is not changing.''

White House spokesman Jay Carney said US President Barack Obama, First Lady Michelle Obama and everyone at the White House wished Mr Mandela a "speedy recovery.''

In a Johannesburg suburb, school children gathered outside Mr Mandela's home yesterday and sang songs expressing hope the former president would recover.

Mr Mandela, the leader of South Africa's anti-apartheid movement, spent 27 years in prison during white racist rule. He was freed in 1990, and then embarked on peacemaking efforts during the tense transition that saw the demise of the apartheid system and his own election as South Africa's first black president in 1994.

His admission to a hospital in Pretoria, the capital, is Mr Mandela's fourth time being admitted to a hospital for treatment since December. His last discharge came April 6 after doctors diagnosed him with pneumonia and drained fluid from his lung area.

At Johannesburg's Apartheid Museum yesterday, visitors walked through an exhibit showcasing the life of Mr Mandela amid a feeling in the country that this hospitalisation may be more serious than previous ones.

"All these admissions to the hospital has been preparing us for this, that this may be the end, and that is enough to tell us this is very serious,'' said Father Victor Phalana of Cathedral of the Sacred Heart in Pretoria, who was touring the museum with two Catholic priests from Uganda.

Outside Mr Mandela's Johannesburg home, school children from the Rainbow Hill Christian School sang words of encouragement.

"We love you Mandela ... get well, get well,'' they sang.

Lebogang Serite, a 12-year-old student at the school, said she "couldn't be in a white people's school'' had it not been for Mandela's anti-apartheid efforts.

"He means a lot to me because he fought for the country. I couldn't be in a white people's school,'' she said.

"I know that if he was able to speak, he was going to play with them today. Unfortunately, wherever he is, he's not well, but I know that he worked very, very hard for us. That's why we are here,'' said Mama Zodwa, a 57-year-old teacher from the school.

On Monday a foundation led by retired archbishop Desmond Tutu described Mandela as an "extraordinary gift'' and offered prayers for his comfort and dignity. The Desmond and Leah Tutu Legacy Foundation described Mr Mandela as "the beloved father of our nation.''

Mr Mandela is seen by many around the world as a symbol of reconciliation because of his peacemaking role when white racist rule ended in South Africa. Archbishop Tutu, 81, was also vigorous campaigner against apartheid, which ended when all-race elections were held in 1994 and Mr Mandela president.

Like Mr Mandela, Mr Tutu was awarded the Nobel Peace Prize for his efforts on behalf of his compatriots. Mr Mandela shared his prize in 1993 with F.W. de Klerk, the last president of the apartheid era.

In its brief statement on Mr Mandela's health, the presidency said Mr Zuma "reiterates his call for South Africa to pray for Madiba and the family during this time,'' referring to Mr Mandela by his clan name.

Tuesday marked 49 years to the day since Mr Mandela was convicted in 1964 for conspiring to overthrow the apartheid government. A day later he was sentenced to life in prison.

He was sent to prison on wind-swept Robben Island, off the coast of Cape Town. He was later transferred to Pollsmoor Prison in Cape Town, where he contracted tuberculosis.

Jailed for 27 years for his beliefs, he became the undisputed face of the anti-apartheid struggle.

Mandela's latest health scare has been met with 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.

He has a long history of lung problems since being diagnosed with early-stage tuberculosis in 1988. This is his fourth hospital stay since December.

Two months ago the Nobel peace laureate, who turns 95 next month, was discharged after treatment for pneumonia.

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

"Pneumonia is a killer disease,'' said 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.''

'No longer the glue holding South Africa together'

In late April, Mr Zuma and top officials in the African National Congress, the anti-apartheid movement turned ruling party, were photographed with an unsmiling Mr Mandela looking exceedingly frail at his Johannesburg home.

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

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

Mr 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,'' said analyst Daniel Silke.

"But I think that this is something long gone, frankly.''

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

Ordinary people, young and old, on Tuesday left messages of support outside his home in northern Johannesburg.

A couple wearing T-shirts bearing the words "We love you Papa Mandela'' placed a teddy bear in a similar shirt outside the gate.

Others wrote messages of support on small stones outside the high security walls, while a group of schoolchildren stopped by to sing for him to "get well''.