India's President Ram Nath Kovind has given assent to the Citizenship (Amendment) bill, effectively turning it into an Act, reports the Times of India.
The act came into effect with the publication of the official gazette on Thursday, December 12.
The bill was earlier passed by the Parliament's lower house Lok Sabha on December 9 and in its upper house Rajya Sabha on December 11.
The act states that members of Hindu, Sikh, Budhhist, Jain, Parsi, and Christian communities who arrived from Pakistan, Bangladesh and Afghanistan up till December 31, 2014 due to religious persecution will be granted citizenship in India.
Refugees of these six communities will be get to be citizens after they have stayed in the country for five years, instead of the earlier requirement of 11.
However, the act will not apply in some of the tribal areas of Assam, Meghalaya, Mizoram stated in the country's constitution in its "sixth schedule".
Protests against the legislation has intensified in the country's northeast, according to the Times of India.
Several towns and cities have been placed under curfew, including Guwahati.
The Indian Union Muslim League (IUML) moved the supreme court, challenging the act and saying that it violates "the fundamental rights to equality in the constitution."