CAPE COAST (Ghana), Aug 27 (AFP) - Ghana's slave castles, some dating back to more than 500 years, attract tens of thousands of visitors every year as well as demands for an apology for centuries of oppression.

Indigation cuts across racial lines as many tourists say that while the past cannot be undone, atonement can lie in gestures like reparation.

The demand has already been voiced by activists in Africa ahead of a UN World Conference Against Racism opening in Durban, South Africa, on August 31. Washington has rejected it.

The Cape Coast castle on the shores of the Atlantic, built as a timber trading lodge by the Swedish in 1652 and captured by the British in 1664, bears mute testimony to centuries of tears, sweat and torture.

Located 120 kilometers (75 miles) west of Accra, it attracts an average of 200 visitors a day, according to officials. Most of the foreign tourists are African Americans.

A plaque at the entrance of the slave dungeons -- almost pitch dark, dank and marked by the desperate scratchings of captured men and women who were kept up to six weeks before being auctioned and sent off to the New World -- pleads that such atrocities are never repeated again by mankind.

"May those who died RIP (rest in peace), May those who return find their roots, May humanity never again perpetuate such injustice against humanity ... We, the living, vow to uphold this."

The prisoners literally wallowed in their excrement in the cells, a guide says, adding that muck could reach up to to 12 inches high during the long wait. Women were routinely raped and abused while "difficult" men were punished, sometimes to death.

Bailus Webb, a retired African-American schoolteacher from Philadelphia, visited Ghana for the second time in two years to visit the slave castles.

"How could anyone treat human beings this way and yet call them savages?" he said. "Who were the real savages?."

Some 10 million Africans were sold into slavery between the 1500s and 1800s, traded by their own kings and local chieftains to European traders for novelties such as alcohol, mirrors and gunpowder.

Webb said he wanted the United States to do three things.

"I strongly support reparation. It is one of the ways healing can take place. It is time America apologises and makes some kind of restitution. We also have to revise the way history is taught in schools as the present syllabus just glosses over slavery."

Katrina Van Buren, a Dutchwoman, was equally appalled.

Pointing to Cape Coast's chapel, which stands just above the male slave dungeons, she said: "They went here and then to church. It makes me sick."

Augustin Adega, a Ghanaian customs officer, backed the demand for reparation.

"If the Germans can pay the Jews why can't the Americans pay us?" he said.

And David Yao Mensah, a guide at the castle, said the tour routinely moved the hordes of visitors, especially African Americans, to tears.

"When they visit the dungeons they don't believe what they see. They cry, they weep. Sometimes fights break out between blacks and whites. One Jamaican woman attacked a white tourist once saying 'You see what your ancestors did. What do you want here? Go away.'"

Ghana tourism official Joel Sonne said a majority of the estimated 400,000 tourists who visited Ghana last year came to the slave castles.

At nearby Elmina castle, built by the Portuguese in 1482 and visited by Christopher Columbus before he discovered the New World, the structures are basically similar to Cape Coast although the dungeons are more commodious and better lit.

It passed into Dutch hands and later to the British. A trap door links the female dungeons to the Dutch governor's bedroom on top of the fort, enabling him to discreetly satiate his physical needs.

Asua Jackson, a Washington-based lawyer, said she was "trying to imagine the terror and the disgust a woman felt when she was being taken to the governor's private chambers ....

"I think every African American should come here to experience this. I was first saddened, then disgusted and then angry."