Obama \'humbled\' by Mandela prison visit

President Barack Obama was \"humbled\" by a visit to the old cell of a now critically ill Nelson Mandela, as he urged a young generation of Africans to take up his hero\'s mantle Sunday. The US leader lauded Mandela and other anti-apartheid inmates of Robben Island who \"refused to yield\" in the face of racist white minority rule, paying homage to the ailing icon he was unable to see in hospital. Obama, accompanied by his wife Michelle and young daughters Sasha and Malia, visited the bleak lime quarry where anti-apartheid activists -- including Mandela -- endured hours of backbreaking toil on this Atlantic outcrop. After staring out the barred window of the small damp cell where Mandela spent two thirds of his 27 years in prison, Obama took a few minutes to write a note in the visitors book.\"On behalf of our family we\'re deeply humbled to stand where men of such courage faced down injustice and refused to yield,\" he wrote. \"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.\" Obama later made Mandela the keystone of an address urging students at the University of Cape Town and elsewhere across the vast continent to make a difference. \"I took my first step in politics because of South Africa,\" he said, recalling his attachment to the anti-apartheid movement in the 1970s as a student in California. Echoing slain US political icon Robert Kennedy\'s call for non-violent change that was made in the same wood panelled hall half a century earlier, Obama urged the students to help shape a continent that is moving forward apace. He recounted Kennedy\'s electrifying words that June day in 1966.\"Each time a man stands up for an ideal, or acts to improve the lot of others, or strikes out against injustice, he sends forth a tiny ripple of hope.\" \"And crossing each other from a million different centres of energy and daring, those ripples build a current which can sweep down the mightiest walls of oppression and resistance.\" Obama said leaders like Mandela and Kennedy were a challenge to him \"but more importantly they stand as a challenge to your generation.\" \"They tell you that your voice matters, your ideas, your willingness to act on those ideas, your choices can make a difference.\" \"If there is any country in the world that shows the power of human beings to affect change, this is the one.\"You have shown us how a prisoner can become a president.\" While Obama acknowledged that Africa is moving forward, he said more work needed to be done. \"There is no question that Africa is on the move, but it is not moving fast enough for the child still languishing in poverty... It\'s not moving fast enough for the protester who is beaten in Harare, for the woman who is raped in eastern Congo,\" he said to a warm reception from the packed auditorium. Obama\'s warm welcome however was not universal. Riot police fired rubber bullets and stun grenades at around 300 hundred anti-Obama protesters on Saturday in the township of Soweto, once a flashpoint in the anti-apartheid struggle. His tour of Senegal, South Africa and Tanzania is aimed at changing perceptions that he has neglected Africa since his election in 2008, while also countering China\'s growing economic influence in the resource-rich continent. As part of that charm offensive he unveiled a $7 billion plan to upgrade African power networks, in bid to end blackouts that deter business investment.He also vowed to host a landmark summit of leaders from across sub-Saharan Africa next year in Washington. But Mandela\'s illness has so far cast a pall over his long-waited Africa trip. It forced him to balance his desire to push for a new economic relationship with Africa, with the need to properly honour his hero, who has been in intensive care for more than three weeks. On Saturday, Obama and his wife Michelle called Mandela\'s wife Graca Machel, and the president then privately visited several daughters and grandchildren of Mandela, to offer support and prayers. But he decided against rolling up in his massive entourage at the Pretoria hospital where the 94-year-old Mandela lies, worried that he would disturb his peace. \"I expressed my hope that Madiba draws peace and comfort from the time that he is spending with loved ones,\" Obama said in a statement using Mandela\'s clan name. Machel said she drew \"strength from the support\" of the Obama family. Mandela, once branded a terrorist by the United States and Britain, was freed in 1990 and became president after the first fully democratic elections in 1994.