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 will go 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.'Heroes'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 will also visit a community project before delivering a keynote address at the University of Cape Town.It is 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 is expected to pay tribute to South Africa's achievements over the past two decades but is expected to stress that more needs to be done to tackle poverty and disease, and strengthen democracy across the continent.He is also due to announce a $7bn (£4.6bn) five-year initiative to double access to electricity in sub-Saharan Africa, in partnership with African countries and the private sector. BeaconMr Obama has been faulted for lacking a grand programme and many Africans have been disappointed at what they see as his lack of engagement with the continent, despite his African ancestry.In Pretoria on Saturday, Mr Obama said Mr Mandela's example of "the power of principle, of people standing up for what's right continues to shine as a beacon".Later, riot police fired rubber bullets and stun grenades at scores of protesters in Soweto, once a flashpoint in the anti-apartheid struggle.At least one person was injured and one arrested."People died in Libya, people are still dying in Syria... in Afghanistan, in Pakistan, drones are still killing people. So that's why we are calling him a Hitler. He's a killer,'' Ramasimong Tsokolibane, 54, was quoted as saying by the Associated Press news agency.Mr Obama arrived in South Africa from Senegal on Friday evening. On Monday, he will continue his African tour in Tanzania.