The United Nations has launched its International Year to Commemorate the Struggle against Slavery.
Unesco wants to encourage teaching about slavery
A ceremony was held in the Ghanaian port of Cape Coast, once one of the most active slave trading centres.
Unesco head Koichiro Matsuura, who is visiting the west African state, said slavery was a tragedy which had remained unrecognised for many years.
But one campaigning organisation cautioned that slavery had not been abolished completely around the world.
The Anti-Slavery International group is working with Unesco, the UN cultural organisation, to raise awareness in schools globally of the transatlantic slave trade, in a programme called Breaking the Silence.
2004 is also the bicentenary of the creation of Haiti, the first black independent state and a symbol of slaves' resistance.
'Door of no return'
Mr Matsuura said the history of the slave trade should take its full place in school textbooks around the world.
"By institutionalising memory, resisting the onset of oblivion, recalling the memory of a tragedy that for long years remained hidden or unrecognised, and by assigning it its proper place in the human conscience, we respond to our duty to remember," he said in a message for the year.
Bonded labour (around 20 million people)
Some forms of child labour (179 million)
Sexual exploitation of children
Trafficking (more than 800,000 people per year - US Government estimate)
Early or forced marriage
Source: Anti-Slavery International
He added that, by remembering the tragic events, the UN wanted to express solidarity and commitment towards those who still did not enjoy basic human rights.
The old castle at Cape Coast, now a World Heritage site and one of Ghana's top tourist attractions, has a "door of no return", through which slaves boarded ships which took them away from their homes and families in Africa.
Mr Matsuura said it had once been a place where human degradation and misery were facts of everyday life.
The contrast between then and now was very sharp, he said, but Cape Coast was full of reminders of the time when human beings were bartered, bought and sold, and then transported far away across the oceans.
Anti-Slavery International's Beth Herzfeld told the BBC that the trade should not be forgotten.
"When we talk about the transatlantic slave trade having a legacy, for instance, xenophobia and racism... that happened for the first time as the result of the transatlantic slave trade, and we are still living with those effects," she said.
"There's no question that the transatlantic slave trade not only affects British society today, but also affects society in the African communities from which people were taken."
Ms Herzfeld added that slavery still existed in most countries around the world, as many people were still, in effect, owned by others.
Chattel slavery, involving a class of hereditary slaves, still existed in parts of Africa, she said, and bonded labour remained common in South Asia.
And new forms of abuse have emerged recently, such as the trafficking of women and girls for the sex trade in Europe.