By Jeff Mason

CAPE COAST, Ghana (Reuters) - After outlining his bright hopes for Africa's future, President Barack Obama got a glimpse on Saturday into one of the darkest chapters of its past -- the transatlantic slave trade.

Obama, America's first black president, took his family for a poignant tour of Cape Coast Castle, a seaside fortress used by slave traders starting in the 17th century and which is now a monument to millions of Africans cast into slavery.

"As painful as it is, I think that it helps to teach all of us that we have to do what we can to fight against the kinds of evils that sadly still exist in our world, not just on this continent but in every corner of the globe," Obama said somberly at end of his visit to the compound.

He likened his tour of the slave castle to his visit last month to the site of the former Nazi concentration camp at Buchenwald in Germany, saying "it reminds us of the capacity of human beings to commit great evil."

But Obama also suggested that from an African American perspective, seeing Cape Coast was bittersweet.

"There's a special sense that on the one hand this place was a place of profound sadness," he told reporters. "On the other hand, it is here where the journey of much of the African American experience began."

Obama, who received a hero's welcome in Accra, flew by helicopter to the site after addressing Ghana's parliament, where he urged Africans to work to rid the impoverished continent of war, corruption and disease and build a more prosperous future.

But the visit to Cape Coast was a look back at a darker era of African history for Obama, the son of a black father from Kenya and a white mother from Kansas.

His wife Michelle, who accompanied him on the tour along with their two daughters, is the descendant of Africans shipped to America as slaves.

While drummers kept up a steady beat outside, Obama and his family toured a dungeon where slaves were kept before being shipped out through a "Door of No Return." The Obamas walked through the door, paused on the other side, then walked back in.

Outside the fortress, thousands of people, some wearing Obama t-shirts and others in native robes, pushed against police barricades to catch a glimpse of Obama's departure. When he stepped out and waved, the crowd cheered wildly.

"Obama's visit is really historic, symbolic. And I think it's also a blessing," said Kwame Sarfo-Adu, 39, a college biology lecturer wearing a t-shirt bearing portraits of Obama and Ghana President John Atta Mills.