The "Global Slavery Index" report by the Walk Free Foundation ranked 162 countries by estimating the number of people in each nation affected by a range of practices including forced and bonded labor, human trafficking, forced marriages, and the use of children in the military.
Walk Free Foundation was founded last year by Australian mining magnate Andrew Forrest. Its index was endorsed by leaders including former U.S. Secretary Hillary Clinton, former British Prime Minister Tony Blair and Microsoft Corp. co-founder Bill Gates.
Clinton said in a statement that although the index wasn't perfect, it provided a starting point.
"I urge leaders around the world to view this index as a call to action, and to stay focused on the work of responding to this crime," she said.
Using reports from governments and non-profit organizations as well as statistical estimates, the charity said that Mauritania has the highest proportion of people in slavery, with many people inheriting slave status from their ancestors. About 140,000 to 160,000 people are enslaved in the west African nation, which has a population of just 3.8 million, the report said.
Haiti came second in the index, with about one in 10 children trapped in an exploitative system of child labor, the report said.
In absolute terms, India, China, Pakistan and Nigeria have the highest numbers of people enslaved, the charity said. Together with five other countries, they account for three quarters of the total estimated number of people in modern slavery.