Messages written to former South African president Nelson Mandela by children from a nearby school are seen on a sign outside Milpark Hospital in Johannesburg, Jan 27, 2011. [Photo/Agencies]

JOHANNESBURG - Former South African President Nelson Mandela was hospitalized for a second straight day Thursday for what his office described as routine tests but which drew concern over the health of the 92-year-old anti-apartheid icon.

Mandela undergoes regular hospital checkups, but his latest visit starting Wednesday stretched into an unusually long stay. Journalists camped outside the hospital Thursday as Mandela's relatives and friends entered for visits. President Jacob Zuma, attending the World Economic Forum annual meeting in Davos, Switzerland, is being updated on developments by the defense minister, whose department is responsible for current and former presidents' health care, Zuma's office said.

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The presidency cautioned reporters not to put pressure on Mandela's doctors and to give "a national hero" dignity and respect.

Children at a school next door to Milpark Hospital made cards and posters wishing Mandela well and put them up on the school grounds Thursday. Head teacher Nicky Humphries said students and teachers prayed for South Africa's most famous and beloved citizen before classes started.

Zuma's office said Mandela "is comfortable and is well looked after by a good team of medical specialists."

Mandela's office has released only a brief statement, saying on Wednesday the hospital visit is for routine tests and that Mandela is in "no danger and is in good spirits." His spokesman did not return calls seeking more information.

The White House said President Barack Obama and first lady Michelle Obama's thoughts are with Mandela.

Mandela was jailed for 27 years for his fight against apartheid. He became South Africa's first black president in 1994 and stepped down after serving one term in 1999. He largely retired from public life in 2004.

The public has seen only glimpses of him recently, such as in November, when his office released photos of a private meeting between Mandela and members of the US and South African soccer teams. The teams had just played a match in his honor. He also appeared at the closing ceremony of the World Cup in July, waving to the crowd as he was driven in a small golf cart alongside his wife, Graca Machel.