About 14 million Indians are living in conditions of modern-day slavery, nearly half of 30 million across the world, according to the first Global Slavery Index published today.
The Global Slavery Index 2013, which surveyed 162 nations, was compiled by Australia-based rights organisation Walk Free Foundation using a definition of modern slavery that includes debt bondage, forced marriage, trafficked into brothels and the use of children in the military.
The Foundation's estimate of 29.8 million slaves worldwide is higher than other attempts to quantify modern slavery. The UN estimates almost 21 million people are victims of forced labour.
India (13,956,010), China (2,949,243), Pakistan (2,127,132) and Nigeria (701,032) have the highest numbers of people enslaved, the rights group claimed.
Together with six other countries, they account for three-quarters of the total estimated number of people enslaved worldwide, it said.
The report said India's ranking was mostly due to the exploitation of Indians citizens within the country itself. It also noted that in India the risk of enslavement varies markedly from state to state.
While the highest proportion of slaves is in Mauritania, with many people inheriting slave status from their ancestors, Haiti is second in the index, Pakistan third and India fourth.
The index, which draws on 10 years of research into slavery conditions around the world and was produced by a team of 4 authors supported by 22 other experts and advisers, is the inaugural edition of what will be an annual report into slavery.
Walk Free policy and research manager Gina Dafalia told CNN that "Our definition of modern slavery includes, for example, forced and servile marriage, a concept not included in the International Labour Organisation estimate, given the focus on 'forced labour'".
Asked why 30 million continued to live in conditions of slavery in 2013, Dafalia said the reasons varied from country to country, but one constant was that it remained a "hidden problem."
Set up in May last year, the Foundation is based in Perth.
It was founded by philanthropist Andrew Forrest, ranked by Forbes as Australia's fifth richest man.
The new index has the backing of noted personalities like former US Secretary of State Hillary Clinton, former British Prime Minister Tony Blair, philanthropists Bill Gates and Richard Branson.