Events are being held worldwide to mark the abolition of the slave trade and to highlight the fact that millions still live as slaves in all but name.
The UN says slavery has taken on new forms
The United Nations is leading the celebrations in Paris, while a new slavery museum is to open in the US state of Ohio later on Monday.
Senegal is holding a commemoration on Goree Island, once a major transit centre for the slave trade.
A UN official said the full extent of slavery had still to be recognised.
The UN's cultural organisation, Unesco, set 23 August as International Day for the Remembrance of the Slave Trade and its Abolition.
It chose the date to commemorate the 1791 San Domingo (Haiti) revolt, which marked the first decisive victory of slaves against their oppressors and led to the creation of the first black independent state.
The UN General Assembly has also proclaimed 2004 as International Year to Commemorate the Struggle against Slavery and its Abolition.
Koichiro Matsuura, head of Unesco, described slavery as an "unprecedented tragedy... concealed for many years and... yet to be fully recognised".
Bonded labour (about 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
"Although abolished and penalised in international instruments, [slavery] is still practised in new forms that today affect millions of men, women and children across the world," he said.
The Anti-Slavery International group - which is working with Unesco to raise awareness in schools globally of the transatlantic slave trade - has described human trafficking as the fastest growing from of modern-day slavery.
The group says the majority of trafficked people are women and girls, and experts believe that most of them are sent from Africa and Eastern Europe for the sex trade in Western Europe.
Anti-Slavery International's Beth Herzfeld told the BBC earlier this year that chattel slavery, involving a class of hereditary slaves, still existed in parts of Africa, and bonded labour remained common in South Asia.
Human trafficking remained a problem in every African country, the UN's Children's Fund, Unicef, said in its April's report.
The report, which covered 53 African nations, said children were the biggest victims, describing how they had been forced into slavery, recruited as child soldiers or sold into prostitution.
Events at Unesco's Paris headquarters on Monday include poetry readings, lectures, a film screening and an exhibition.
Ohio is opening a museum dedicated to slaves and anti-slavery activists, focusing on the "Underground Railroad" - the clandestine network which existed up to the American Civil War to help captives escape to freedom.
A grand opening ceremony for the Freedom Center's museum will include a special concert later on Monday.
"We want to provide a powerful experience that demonstrates how the conviction, dedication and passion of some very diverse people changed the world 150 years ago," Spencer Crew, executive director of the National Underground Railroad Freedom Center, told AFP news agency.
Commemorative ceremonies are also being held in Senegal's Goree Island, where captives were held on the way to the Americas during the transatlantic slave trade.
The Goree Island is now a World Heritage site.
MODERN SLAVERY: A GLOBAL PROBLEM
(Slavery is a problem around the world and very difficult to measure. The following examples are listed in geographical order and not in terms of the magnitude of the problem.)
1. US: An estimated 20,000 people are trafficked into the US annually - many are forced into prostitution.
2. Dominican Republic: Campaigners say hundreds of thousands of Haitians are rounded up near the border and made to work on Dominican sugar plantations.
3. Brazil: Up to 25,000 people are said to be working as slave labourers - most of them clearing Amazonian forests.
4. Mauritania: Despite its abolition in 1981, chattel slavery is still strong - up to 1m people are allegedly held as "inheritable property".
5. Sudan: Campaigners say northern militias continue to take women and children in slave raids in the south.
6. Europe: Tens of thousands of women and girls are cheated, abducted and forced into prostitution right across Europe.
7. UAE: Every year hundreds of boys are reportedly trafficked from South Asia to the UAE and other Gulf states to race camels.
8. Pakistan: Men, women and children are bonded into forced labour in agriculture and industry, campaigners say.
9. Burma: Forced labour is reportedly used on a growing number infrastructure projects.
10. Thailand: Thousands of girls are sex slaves for tourists.
(Sources: ILO, American Anti-Slavery Group, US state department.)