Obama 'humbled' to visit Mandela's Robben Island jail

US President Barack Obama has toured Robben Island - the jail in which Nelson Mandela was kept for 18 years.

He said he and his family were "deeply humbled" to visit the prison once inhabited by Mr Mandela - who remains critically ill in hospital.

Mr Obama went on to give a speech at the University of Cape Town and launch a multi-billion-dollar electricity initiative.

The US leader did not visit Mr Mandela, but met the Mandela family in private.

Security is likely to have been strengthened during this final Cape Town leg of his time in South Africa, says the BBC's Karen Allen who is there, following clashes on Saturday between riot police and anti-Obama protesters in Soweto.

Mr Obama and the first family visited Mr Mandela's bleak cell as well as the lime quarry - overlooked by a concrete watchtower - where anti-apartheid fighters including Mr Mandela were forced to undertake hours of back-breaking labour.

Mr Mandela was at the prison for 18 years and his long history of lung problems can be traced to the tuberculosis he contracted there - which he attributed to the dampness of his cell.

Later, Mr Obama wrote in the guest book in the prison courtyard: "On behalf of our family, we're deeply humbled to stand where men of such courage faced down injustice and refused to yield.

"The world is grateful for the heroes of Robben Island, who remind us that no shackles or cells can match the strength of the human spirit."

Mr Obama also visited a community project before delivering a keynote address at the University of Cape Town.

It was the same venue where 47 years ago, US Senator Robert Kennedy gave his famed "ripple of hope" speech, which gave inspiration to those fighting the racially divisive policies of apartheid rule and linked their struggle with that of the US civil rights movement.

Mr Obama paid tribute to South Africa's achievements over the past two decades but urged young Africans to fulfil Mr Mandela's legacy.

"Nelson Mandela showed us that one man's courage can move the world,'' he said.

More needed to be done to tackle poverty and disease, he said, adding that fear too often prevailed in Africa. For as long as war raged, democracy and economic opportunity could not take hold, he said.