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U.S. President Barack Obama stands in the Robben Island prison cell where former South African President Nelson Mandela spent 18 of his 27 years of imprisonment, June 30, 2013. Reuters / Jason Reed

President Obama continued his South Africa tour on Sunday while former South African president Nelson Mandela remained in hospital care in Pretoria.

One day after meeting with Mandela's family members on Saturday at the Nelson Mandela Centre of Memory, the U.S. President and the first family, accompanied by South African President Jacob Zuma, flew by helicopter to the Robben Island prison off Cape Town's coast, where South Africa's apartheid government held Mandela for 27 years.

They were guided by former inmate and anti-apartheid activist Ahmed Kathrada, 83, who spoke about his time in the prison with Mandela and other African National Congress prisoners.

Obama previously visited Mandela's cell in 2006 during an Africa tour while he was a U.S. senator.

"On behalf of our family we're deeply humbled to stand where men of such courage faced down injustice and refused to yield," Obama wrote in the landmark's guestbook. "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."

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U.S. President Barack Obama and his family listen to Robben Island prison guide Ahmed Kathrada (2nd from right), who was an inmate with Nelson Mandela at the prison near Cape Town, on June 30, 2013. Pictured are Obama's family members: niece Leslie Robinson, daughters Malia and Sasha, first lady Michelle Obama and Michelle's mother, Marian Robinson. Reuters / Gary Cameron
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U.S. President Barack Obama stands with his family as they visit the rock quarry labor camp where Nelson Mandela was forced to work, during their tour of Robben Island Prison near Cape Town, June 30, 2013. Reuters / Jason Reed
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U.S. President Barack Obama is escorted to Nelson Mandela's cell by former Robben Island Prison inmate and now-tour guide Ahmed Kathrada (R), who was imprisoned in the facility at the same time as Mandela, near Cape Town, June 30, 2013. Reuters / Gary Cameron
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U.S. President Barack Obama and first lady Michelle Obama enter the prison on Robben Island near Cape Town, where Nelson Mandela was imprisoned for 27 years, June 30, 2013. Reuters / Jason Reed
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U.S. President Barack Obama and first lady Michelle Obama approach the doors of the prison on Robben Island near Cape Town, where Nelson Mandela was imprisoned for 27 years, June 30, 2013. Reuters / Jason Reed
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U.S. President Barack Obama stands in the Robben Island prison cell in which former South African President Nelson Mandela spent 18 of his 27 years of imprisonment, June 30, 2013. Reuteres / Jason Reed
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U.S. President Barack Obama and First lady Michelle Obama exit the cell block where Nelson Mandela was held captive on Robben Island near Cape Town, June 30, 2013. Reuters / Jason Reed
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U.S. President Barack Obama writes in a guest book as he tours Robben Island prison with first lady Michelle Obama, near Cape Town, June 30, 2013. Under apartheid, Nelson Mandela spent several decades as a political prisoner on Robben Island. Reuters / Jason Reed
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A hand-written message by U.S. President Barack Obama is pictured in the prison's guestbook as he and First Lady Michelle Obama toured Robben Island near Cape Town, June 30, 2013. "On behalf of our family we're deeply humbled to stand where men of such courage faced down injustice and refused to yield," Obama 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." Reuters / Jason Reed
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South Africa Jacob zuma Michelle Obama Nelson mandela