In India slavery is often disguised. In popular discourse it is termed 'bonded labour'. The Global Slavery Index 2013 says India has the highest number of people living in conditions of slavery at 14 million (estimated number of slaves 13,956,010). The index is compiled by Australian-based rights organisation Walk Free Foundation using a definition of modern slavery that includes debt bondage, forced marriage and human trafficking.
India, China, Pakistan and Nigeria have the highest numbers of people enslaved. By proportional ranking India comes fourth, after Mauritania, Haiti and Pakistan. The report said India's ranking was mostly due to the exploitation of Indians citizens within the country itself. The report estimates nearly 30 million people around the world are living as slaves, ranking 162 countries. The International Labour Organisation estimates that almost 21 million people in the world are victims of forced labour. (The difference in estimates are due to difference in definition). You can read more on the report here and see the entire report with interactive map here.