Barack Obama has spoken of how he and his family had been "deeply humbled" to stand in the cell where Nelson Mandela was held for 18 years as South Africa's most dangerous political prisoner.
The US president and his wife Michelle took daughters Sasha and Malia to the museum created from the former prison island off the coast of Cape Town for what he said he hoped would be "an experience they would never forget".
They were guided by Mr Mandela's close friend and former co-detainee, Ahmed Kathrada, through the bleak stone quarry where they and the 32 other anti-apartheid activists were forced to work before being taken in to Prisoner 46664's tiny cell.
Standing in the darkened room where Mr Mandela was kept hidden away from the world for so long, Mr Obama gazed through the barred window before picking up a small metal bowl he would have eaten from and looking down to the thin straw mat on the floor where he would have slept.
Afterwards, he and his wife wrote a tribute in the museum visitor's book to Mr Mandela, who now lies critically ill in a Pretoria hospital, and his fellow prisoners.
"On behalf of our family we're deeply humbled to stand where men of such courage faced down injustice and refused to yield," their entry read.
"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."
In a recreation yard paved with concrete and dotted with shrubs, the Obamas were shown where Mr Mandela buried pieces of the manuscript of his autobiography, 'Long Walk to Freedom'. The manuscript was eventually smuggled out of Robben Island by Mac Maharaj, another jailed political activist who now serves as President Jacob Zuma's spokesman, delivering updates to the nation about the fading health of his old friend Mr Mandela.
There have been no updates on Mr Mandela's condition since Mr Zuma said he remained "critical but stable" during a joint press conference with Mr Obama on Saturday morning. He is now entering his fourth week in the hospital where he was rushed early on Saturday June 8, suffering from a recurring lung infection.
Embarking on the main speech of his long-awaited Africa tour at the University of Cape Town later, Mr Obama said he had visited Robben Island before when he was a senator but his latest visit held far more resonance.
"There was something different about bringing my children, Malia is now 15, Sasha is 12," he said.
"Seeing them stand within the walls that once surrounded Nelson Mandela, I knew this was an experience they would never forget, that they now appreciated a little bit more the sacrifices Madiba and his contemporaries made for freedom.
"Mandela's spirit could never be imprisoned because his legacy is here for all to see . . . (he) shows us that one man's courage can move the world."
As the Obamas toured Robben Island, it was noted on Twitter that it was "ironic" the US president should lament the cruelty of that institution while America's own prison island, Guantanamo Bay – which he pledged to close before his election – remains functional.
As Mr Obama visited a HIV centre later, the point was made more forcefully by Archbishop Desmond Tutu. "We pray you will be known as having brought peace to all the regions where there has been strife and peace and no need for Guantanamo Bay," the former Archbishop of Cape Town told him.
He said the arrival on the continent of Mr Obama, who moves on to Tanzania today, held a "special joy" for him.
"When you became the first black incumbent of the White House you don't know what you did for our psyche," he said. "My wife sat in front of a TV with tears running down her face . . . So welcome home, even if you're about to go." (© Daily Telegraph, London)