The Archbishops of Canterbury and York have used an video-sharing website to highlight the history of slavery.
Dr Sentamu said he was left "speechless" by what he had seen
They have posted the footage of their trip to Zanzibar on YouTube to coincide with the 200th anniversary of the abolition of the slave trade.
Dr Rowan Williams and Dr John Sentamu are shown visiting two preserved slave pits where up to 175 men, women and children were kept chained up.
The video was recorded last month and also reflects on modern-day slavery.
During the trip, Dr Sentamu also spent some time at a memorial to the slaves which features some of the original chains used when the market was operating.
He said: "I found the whole experience heart-rending.
"When I went outside and actually saw those figures - how slaves were tied together - and touched the actual chains that were used, I was rendered absolutely speechless.
"I felt I was going back in history, but I was also in the present where still, slavery in some parts of the world still happens."
In the film, Dr Williams said that 200 years on, slavery was still a problem in the world.
He said: "It's as if slavery is a kind of compulsion for human societies.
"People go back again and again to treating people as objects, as possessions, and I don't think we can simply sit back and say it's a thing of the past and no more."
"All those modern forms of slavery: economic slavery - debt slavery in effect - the slavery of sex trafficking, these things are still with us."