File photo taken on Feb. 3, 2005 showing South African former president Nelson Mandela during a speech in Trafagar Square, London.CORBIS

Nelson Mandela was "responding positively" to treatment Thursday after being readmitted to hospital with a lung infection, the latest health scare for the much-loved anti-apartheid icon.

The frail 94-year-old former South African president was hospitalized just before midnight on Wednesday, the presidency said, urging people around the world to pray for him.

"The doctors advise that former president Nelson Mandela is responding positively to the treatment he is undergoing for a recurring lung infection," President Jacob Zuma's office said in a short statement.

"He remains under treatment and observation in hospital."

The Nobel peace laureate, regarded as the father of the "rainbow nation", was conscious when he was admitted, presidency spokesman Mac Maharaj told AFP.

It is the second time this month that Mandela has spent the night in hospital and follows a nearly three-week stay in December for the lung infection and for gallstone surgery, after which he was released for home-based care.

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Mandela was diagnosed with early-stage tuberculosis in 1988 during his near three-decades in prison under the white-minority apartheid regime and has long problems with his lungs.

"Doctors are attending to him, ensuring that he has the best possible expert medical treatment and comfort," Zuma's office said earlier.

Zuma wished "Madiba", as South Africa's first black president is affectionately known, a quick recovery and asked for people around the world to pray for him.

"We appeal to the people of South Africa and the world to pray for our beloved Madiba and his family and to keep them in their thoughts," he said.

"We have full confidence in the medical team and know that they will do everything possible to ensure recovery."

Revered at home and abroad as the symbol of the country's peaceful shift to democracy after apartheid, his "long walk to freedom" in 1990 after 27 years behind bars was one of the most potent images of the time.

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But as he has grown increasingly frail, he has not appeared in public since South Africa's football World Cup final in 2010, six years after retiring from public life.

The ruling African National Congress, the once-banned liberation movement that Mandela led into power, serving as president for one term from 1994, also called for prayers for their former leader.

"During these trying times we wish President Mandela well and for his family to be strong," the party said in a statement, adding that it was "confident that the treatment will be successful".

The name and location of the hospital treating Mandela were not disclosed, to allow the medical team to focus on their work and to shield the family from the intense media interest.

"We know they are going through a difficult time and we want to ensure that their privacy is maintained," Maharaj said.

Mandela's December hospital stay was his longest since he walked free in 1990.

Early last year, he was admitted for a minor exploratory procedure to investigate persistent abdominal pain. In 2011, he was hospitalized for two nights for an unnamed acute respiratory infection.

News of his latest ill health was slow to reach Qunu, his rural childhood Eastern Cape village where he had spent recent years before leaving in December.

"Most of the people in the village don't know even that he is in hospital," Zimsile Gamakulu, a local guide and also of the Madiba clan, told AFP, adding that villagers "wish him a long life".

"They miss him a lot, especially the older ones," he added. "They hope that he may come back home."

In February, Zuma said he found Mandela "comfortable and relaxed" and watching television at his Johannesburg home.

"He had the brightest smile," said Zuma.

Earlier this month, his friend and renowned human rights lawyer George Bizos, who defended Mandela during his 1960s treason trial, said the ex-president was aware of current political events but was having some trouble with his memory.

"Unfortunately he sometimes forgets that one or two of them had passed on and has a blank face when you tell him that Walter Sisulu and some others are no longer with us," Bizos said.

Sisulu, a former ANC leader who was Mandela's political mentor, died nearly a decade ago.

Last month, two of his granddaughters released a picture of a smiling Mandela sitting with his youngest great-grandson in an armchair.