Britain's Prince Charles addresses a young entrepreneurs event Monday in Accra, Ghana.
Accra, Ghana CNN  — 

Britain’s Prince Charles spoke out strongly against modern slavery at a conference in Ghana, once the embarkation point for many Africans traded as slaves.

Charles, on the second leg of a West African tour, said Monday it was unimaginable the suffering the trans-Atlantic slave trade caused and that it left an “indelible stain on the history of our world.”

British traders were among the largest participants in the Atlantic slave trade in which up to 12 million Africans were estimated to have been sold into slavery. Britain abolished the trans-Atlantic slave trade in 1807.

“Britain can be proud that it later led the way in the abolition of this shameful trade,” said Charles, the heir apparent to the British throne. “We have a shared responsibility to ensure that the abject horror of slavery is never forgotten, that we abhor the existence of modern slavery.”

The Prince of Wales and his wife, Camilla, the Duchess of Cornwall, visited Osu Castle, a one-time slave fort in Accra, on the Atlantic coast’s Gulf of Guinea.

Built in 1661, the castle was a principal holding cell for enslaved Africans during the trans-Atlantic slave trade. It also was the headquarters for the British colonial government.

The duchess also stopped by the Ghana International School to highlight the Commonwealth Essay Competition prize, an international writing competition that the Royal Commonwealth Society has managed since 1883.

On Monday night, Ghanaian President Nana Akufo-Addo held a state dinner to celebrate the British royals’ visit.

In Gambia, the Prince congratulated the West African nation on recently returning to the 53-member Commonwealth states of mostly former British territories. He was tapped earlier this year to succeed Queen Elizabeth II as the head of the Commonwealth.

The royal couple paid their respects to those who lost their lives in World War I and World War II at a Commonwealth War Graves Commission cemetery.

On Tuesday, Charles and Camilla arrived in Nigeria, where President Muhammadu Buhari welcomed them at the Presidential Villa.

Charles’ last trip to Nigeria was in 2006, but Camilla is visiting Africa’s most populous nation for the first time.

CNN’s David Wilkinson and Max Foster reported in Accra, while Damilola Odutayo wrote in Lagos, Nigeria.