US President Barack Obama in Nelson Mandela's old cell
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Obama 'humbled' by Robben Island

US President Obama has described a visit to Robben Island, where Nelson Mandela spent 18 of his 27 years in prison, as deeply humbling, writing in the visitors' book that "the world is grateful for the heroes of Robben Island."

Speaking later at the University of Cape Town, Mr Obama said he had been inspired to go into politics by the struggle in South Africa.

Meanwhile, the 94-year-old former President Nelson Mandela remains critically ill in hospital.

Karen Allen reports from Cape Town.