President Barack Obama visits Robben Island jail in the final leg of South African tour

Obama said he was 'deeply humbled' to stand 'where men of such courage faced down injustice'

The president has chosen historically resonant locations for the address, and will speak at the University of Cape Town after touring the prison on Robben Island.

US President Barack Obama visited Robben Island jail on Sunday, where Nelson Mandela was imprisoned for nearly two decades.

Mr Obama is coming to the end of his tour of South Africa and met with President Jacob Zuma on Saturday to pay tribute to the Nobel Peace Laureate.

He also met with the anti-apartheid leader’s family on Saturday before speaking with Mr Mandela’s wife Graca Machel by phone, offering his “heartfelt support”.

Mandela, 94, remains in a critical condition after being hospitalised with a recurring lung infection.

The White House said Mr Obama's guide during the tour was 83-year-old South African politician Ahmed Kathrada, who also was held at the prison for nearly two decades and guided Mr Obama on his 2006 visit to the prison as a US senator.

The president was shown the prison courtyard where Mandela planted grapevines that remain today, and where he and others in the dissident leadership would speak.

Mr Mandela was held prisoner in Robben Island for 18 years until he was transferred to Pollsmoor Prison in 1982, before eventually securing his release from Victor Verster Prison in 1990. It was in his damp Robben Island cell that Mr Mandela first contracted tuberculosis.

Mr Obama wrote in the guest book in the 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."

After the tour, the president visited retired archbishop Desmond Tutu at a youth centre run by his HIV foundation before delivering what the White House has billed as the signature speech of the president's weeklong trip, an address at the University of Cape Town, where Robert F. Kennedy delivered his famous "Ripple of Hope" speech. Kennedy spoke in Cape Town two years after Mandela was sentenced to life in prison.

He will call on young Africans to summon similar energy to complete the work of those movements and to firmly establish economic growth, democratic government, and stable societies across the continent.

The president's trip has taken him to Senegal and South Africa and will wind up in Tanzania on Tuesday.

Although concerns over the ailing health of Mr Mandela have overshadowed much of the trip, the president has discussed Africa's economic potential at every stop.

In keeping with that emphasis, Mr Obama will also announce that he plans to hold a summit of sub-Saharan African leaders in Washington next year.

Additional reporting by agencies