a system of child labour known as "restavek" encourages poor families to send their children to wealthier acquaintances, where many end up exploited and abused. Pakistan, India, Nepal, Moldova, Benin, Ivory Coast, Gambia and Gabon have the next highest prevalence rates.
At the other end of the scale, Iceland has the lowest estimated prevalence with fewer than 100 slaves.
Next best are Ireland, Britain, New Zealand, Switzerland, Sweden, Norway, Luxembourg, Finland and Denmark, although researchers said slave numbers in such wealthy countries were higher than previously thought.
"They've been allocating resources against this crime according to the tiny handful of cases that they've been aware of," said Kevin Bales, lead researcher and a professor at the Wilberforce Institute for the Study of Slavery and Emancipation at Hull University.
"Our estimates are telling them that the numbers of people in slavery - whether it's in Great Britain or Finland or wherever - in these richer countries actually tends to be about six to 10 times higher than they think it is."
Walk Free CEO Nick Grono said the annual index would serve as an important baseline for governments and activists in the anti-slavery fight.
"This kind of data hasn't been out there before," he said. "It's a multi-year effort, and next year we'll have a much better picture of where slavery is and what changes there are. If you can't measure it, you can't devise policy to address it."