Nelson Mandela in hospital for routine tests

Mandela was admitted to a Johannesburg hospital (GETTY)

The former South African President Nelson Mandela is in hospital for routine medical tests, reigniting fears over the health of the frail 92-year-old anti-apartheid icon.

Mandela was admitted to a Johannesburg hospital yesterday for what his foundation described as routine tests but Talk Radio 702 reported that Mandela had been seen by a specialist pulmonologist, who treats respiratory systems.

South Africa's ruling African National Congress appealed for calm today after the hospitalisation sparked a media frenzy and much speculation over Mandela's health in local newspapers.

"He is a 92-year-old and will have ailments associated with his age and the fact that he stayed the night should not suggest the worst," ANC spokesman Jackson Mthembu said.

"We urge people not to make unfounded statements, let's remain calm and not press panic buttons because there is no reason to do so."

Mandela has not been seen in public since the final of the soccer World Cup in July last year.

Police were called in to control traffic at the Milpark Hospital in a leafy Johannesburg suburb as scores of journalists, photographers and television crews converged on the institution.

Several of Mandela's family members, including his wife Graca Machel, visited the hospital after his admission, Reuters witnesses said.

Mandela retired from public life in June 2004 ahead of his 86th birthday, telling his adoring compatriots: "Don't call me, I'll call you."

Since then he has rarely appeared in public and when he did, appeared increasingly frail.

South African Nobel Peace Prize laureate Desmond Tutu said this week he met Mandela last week.

"He was all right, I mean, he's 92, you know. And he's frail."

Mandela - known by his clan name of Madiba in South Africa - has been on holiday with his wife, local media reported.

Earlier this month the ANC said it wanted Twitter to take action after a hoax report of Mandela's death was widely distributed on the social network site.

The Nelson Mandela Foundation declined to comment on his health on Thursday but said in an earlier statement: "He is in no danger and is in good spirits." According to Internet website Trendistic, which tracks Twitter trends, the term "Mandela" has increased sharply over the past 24 hours. http://trendistic.com/mandela/—24-hours

Concerned South Africans also used social network site Facebook to post their concern. "All the best for you! hope (your) dad is just fine!," wrote one person on the Facebook page of Mandela's daughter, Zinzi.