The event's organizers said the law in Tamil Nadu state prevents Dalits, many of whom are known as Untouchables, from escaping Hindu caste discrimination that survives despite being officially outlawed for decades.
Many Dalits have turned to Christianity, Buddhism or Islam to escape the 2,500-year-old caste system. Hundreds gathered at Friday's ceremony to cheer the new converts.
Tamil Nadu passed a law last month imposing penalties of up to three years in prison for converting someone by force or inducement. Hard-line Hindus accuse missionaries from other religions of luring poor people away from Hinduism - India's largest faith - through offers of money or jobs, or through coercion.
The leaders of minority religions deny such claims, and say the new law violates religious freedoms guaranteed by India's constitution and the U.N. Human Rights Charter.
Minority religious leaders fear the term "inducement" could apply to the free education and health care they provide.