President George W Bush has described the transatlantic slave trade as "one of the greatest crimes of history".
Bush came close to apologising
The president, speaking at the start of a five-nation tour of Africa, said: "Liberty and life were stolen and sold."
He delivered the speech on Goree Island in Senegal, from where hundreds of thousands of Africans were sent in shackles to America between the 16th and 19th Centuries.
The president paid tribute to African leaders like Nelson Mandela and a number of prominent black Americans who had helped "awaken the conscience of America".
The BBC's Matt Frei, who is travelling with the US president, said Mr Bush came close to an outright apology for slavery, which one of his senior advisers called America's birth defect.
"America learned that freedom is not the possession of one race," Mr Bush said, pledging to work as "equal partners" with African nations to improve trade ties, fight terrorism and tackle Aids.
He said America's power and resources committed it to further the cause of freedom in Africa, too.
Mr Bush has already pledged to spend $15bn on fighting the spread of Aids, which is ravaging Africa.
He also wants to promote economic transparency - $10bn in aid is available to countries which promise to fight corruption and open their markets.
But with US presidential elections looming, our correspondent says Mr Bush remains committed to the farm subsidies at home which have crippled cheap African exports to America.
And Congress in Washington is already planning to cut back the money allocated to the plans - an estimated $1bn less for the next fiscal year.
The US leader has not said if and when American peacekeeping troops will be sent to end the fighting in nearby Liberia - an issue overshadowing the visit.
Mr Bush said Washington had not yet decided whether to send peacekeepers to help end the conflict in Liberia.
At the same time, he said it would work with the United Nations and western African states to maintain the current ceasefire in the country.
From Senegal, Mr Bush travelled to South Africa - the second leg of his tour which will also take him to Botswana, Uganda and Nigeria.
Nigeria has led West African nations in urging the US to contribute up to 2,000 troops to a task force for Liberia which would also include 3,000 African soldiers.
Mr Bush is not popular in Africa, particularly since his invasion of Iraq.
Ahead of his arrival in Senegal, protesters took to the streets to denounce him as a "butcher".
But by Tuesday morning, Mr Bush was able to travel through a deserted city because the Senegalese authorities had cleared the area - closing off streets and removing traders and traffic.
Many people on Goree Island were asked to leave their homes for the duration of Mr Bush's visit - sparking anger in the community.
The BBC's West Africa correspondent, Paul Welsh, says politicians in Senegal believe Mr Bush is more interested in photo opportunities to defend his interests at home than in West Africa's problems.
Some believe the tour is aimed primarily at winning the support of African American voters - who traditionally vote Democrat - in next year's US presidential election.
In South Africa - as in the other countries on his itinerary - Mr Bush is expected to focus on trade, terrorism, and the fight against Aids.
South Africa has the largest HIV-infected population in the world.