Students posted get well cards on the gates of a Johannesburg school [EPA]

South Africans are awaiting news about the health of Nelson Mandela, the former South African president, who has been admitted to a Johannesburg hospital.

The ruling African National Congress (ANC), called for calm on Thursday, while the Nelson Mandela Foundation said the former president was in Milpark Hospital for "routine tests" but that his health was not in jeopardy.

"We call on all South Africans to remain calm regarding the hospitalisation of Madiba and not press any panic buttons, as there is no reason for that whatsoever," African National Congress spokesman Jackson Mthembu said in a statement.

The news has put South Africa on edge over the health of the increasingly frail "Madiba" - the clan name by which the 92-year-old Nobel peace prize winner is affectionately known.

The Star newspaper reported Thursday that Mandela had been seen by a lung specialist at the private hospital.

"He has been admitted for investigation," the doctor, Michael Plit, told the newspaper. He declined to comment on Mandela's condition.

Mandela's wife, Graca Machel, and other family members were seen at the hospital on Wednesday night. Machel's daughter Josina and Mandela's personal assistant, Zelda la Grange, were at the hospital on Thursday morning.

Winnie Madikizela-Mandela, his former wife, also visited him in the hospital on Thursday.

As media flocked to the hospital for news on Mandela's health, a tight security presence surrounded the building - with police checking all visitors' cars to make sure no journalists were hiding in the boot.

Students pray

At a school next door to the hospital, children had decorated a fence with colourful pictures of hands and hearts and messages of support.

"We hope you'll get well soon," read one.

"Madiba, we love you," read a sign in one of the school's windows.

Ntho Molena, a 16-year-old school pupil, said she and her colleagues were praying for Mandela to get well.

Mandela spent 27 years in prison for his role in the fight against Apartheid in South Africa, emerging in 1990 to lead the country's transition to democracy.

As South Africa's first black president, he defied the threat of civil war to lead a process of reconciliation in a country long divided against itself.

Mandela's public appearances have become increasingly rare since his retirement.

His last public outing was at the closing ceremony of the 2010 FIFA World Cup in Johannesburg.