A new report from the Walk Free Foundation says an estimated 30 million people are enslaved around the world.
The Australia-based group presented its findings Thursday in its first Global Slavery Index, a ranking of 162 countries by their prevalence of modern slavery.
In compiling the index, researchers considered crimes such as human trafficking, forced labor and the exploitation of children.
Mauritania is the nation with the highest prevalence of modern slavery.
The foundation says the West African nation has a long history of hereditary slavery based on ethnicity. The group says one report indicates up to 20 percent of the Mauritanian population is enslaved.
On the index, Mauritania is followed by Haiti, Pakistan, India and Nepal.
Foundation researcher Kevin Bales says the report is designed to raise awareness about the plight of modern slaves.
In a VOA interview, he said many of the people who wind up in slave-like conditions are "economic hopeful" migrants. He says criminals take advantage of these migrants, who are seeking better lives for themselves and their families.
African nations comprise half of the group's top 10 rankings on index by prevalence.
In addition to Mauritania, Benin is cited for the trafficking of women and children for sexual exploitation or forced labor. It ranks seventh on the index.
Benin is followed by Ivory Coast. The group says the country is a "source, transit and destination country" for women and children subjected to forced labor and sexual exploitation.
Rounding out the top 10 are The Gambia and Gabon. The Gambia is cited for problems that include forced begging and child marriages. The group says problems in Gabon include a cross-border trafficking of children.
In a separate index, the foundation ranks India, China and Pakistan as the countries with the highest numbers of enslaved people within their populations.
The foundation says the role of government in addressing modern slavery, a human rights violation, is "paramount." The group says only governments have the authority to enact and enforce laws against slavery.