Almost all African states are affected by trafficking as importers or exporters, or both, a United Nations Children's Fund report says.
It also warns that children are the biggest victims of trafficking: forced into slavery, recruited as child soldiers or sold into prostitution.
BBC reporters in three countries explore the illicit trade - from its origins in Africa to its destinations in Europe.
Anna Borzello in Lagos, Nigeria
Thousands of Nigerian children and young people are trafficked each year, both within the country and beyond its borders.
No-one knows the scale of the problem, but the International Labour Organisation estimates at least 200,000 children are trafficked within west Africa annually.
In the eastern Nigerian city of Benin, girls and young women are sent to west Africa and Europe to work as prostitutes.
Many go willingly, but some are tricked.
They are taken to a witch doctor, sworn to secrecy and end up in effective slavery to the middle men who smuggle them abroad.
Other children are trafficked for work inside the country.
Girls and boys as young as six are taken from desperately poor homes and placed as domestic workers with strangers in the city.
In return, they are promised an education. In reality, they are often beaten, fed on leftovers, forced to work long hours and forbidden to go to school.
Other children are sent instead to work in quarries or plantations, both inside Nigeria and in neighbouring west African states.
Some are even trafficked for ritual purposes and end up dead.
The Nigerian government has now recognised the problem and last year outlawed trafficking and set up an agency to deal with offenders.
But they have a difficult task ahead. Close to 70% of the population live in poverty and traditional values have been eroded by years of misrule.
For many Nigerians, trafficking has become an accepted way of life.
Pascale Harter from Rabat, Morocco
In Morocco and in neighbouring Mauritania women and children are often treated as commodities.
There are still children being born into slavery in Africa
While reporting from Mauritania, I met a runaway slave who told me she had been raped by the men of her master's family.
Her children are still in his custody, born into slavery just as she and her mother was before her.
Slavery was officially abolished in Mauritania in the 1980s, but local associations say many families still have slaves, who they sometimes send to business associates to work off a debt.
Now in her late 20s, the runaway slave I spoke to had started working for her master as soon as she was old enough to carry water and clean.
But she has never earned a day's pay.
In Morocco the problem is different. Here many children are born into poverty.
Abandoned by their husbands and unable to find work, women are forced to send their children from the age of five to be apprentices to tailors or carpet weavers, where at least they will be fed.
At the port of Casablanca, Moroccan children as young as seven can be seen running behind trucks carrying goods to Europe.
They jump on to the back of the trucks and hide there. Despite the danger of the journey, they often leave with the consent of their parents who are simply to poor to provide for them.
Frances Kennedy from Rome, Italy
Many of the African girls and women who are trafficked to Europe end up on the streets.
Many African girls are lured abroad by false offers of jobs
Italy is one of their prime destinations. There are tens of thousands of foreign prostitutes on Italian streets - many of them African and particularly Nigerian, and some just girls.
Promises of jobs as babysitters or waitresses evaporate as the women arrive and are put to work.
In the suburbs of main cities and on provincial highways, rain or shine, scantily-clad African women are on sale.
Italian men like them because they are inexpensive and obliging.
They are closely controlled by a "maman", an older woman, and those who fail to earn enough are punished with violence.
They are deprived of their passports and some of them are held as virtual prisoners.
Programmes to encourage African girls out of prostitution have had little success for several reasons:
- their exploiters threaten to harm their families back in Africa
- the women are convinced through voodoo-type rituals that their own lives will be at risk if they break free
- they worry they won't be able to find legal work to keep supporting their relatives back home.