India's Hindu nationalist government on Wednesday won parliamentary approval for a far-reaching citizenship law that critics say undermines the country's secular constitution by excluding Muslims.
Violence broke out in the ethnically diverse northeast while parliament debated the bill, as protesters against the legislation clashed with police.
It is the third key election promise that Prime Minister Narendra Modi's government has delivered since he was re-elected in May, re-energizing his nationalist, Hindu support base and drawing attention away from a slackening economy.
Soldiers were deployed in Tripura state and reinforcements put on standby in neighboring Assam, both of which border Bangladesh, where people fear an influx of settlers.
"The bill will take away our rights, language and culture with millions of Bangladeshis getting citizenship," Gitimoni Dutta, a college student, said at a protest in Assam's main city of Guwahati.
The Citizenship Amendment Bill seeks to protect besieged minorities in neighboring states by granting Indian nationality to Buddhists, Christians, Hindus, Jains, Parsis and Sikhs who fled Afghanistan, Bangladesh and Pakistan before 2015.
The bill passed the upper house of parliament with 125 members supporting it and 105 opposing.
The move by Modi's government faced stiff resistance from opposition parties, minority groups and student bodies.
Despite assurances from India's Home Minister Amit Shah that safeguards will be put in place, people in Assam and surrounding states fear that arriving settlers could increase competition for land and upset the region's demographic balance.
Some opposition Muslim politicians have also argued that the bill targets their community, which numbers more than 170 million people and is by far India's largest minority group.
The government has said the new law will be followed by a citizenship register that means Muslims must prove they were original residents of India and not refugees from these three countries, potentially rendering some of them stateless.
Members of other faiths listed in the new law, by contrast, have a clear path to citizenship.
The U.S. Commission on International Religious Freedom said on Monday that Washington should consider sanctions against Shah, a close associate of Modi, if India adopts the legislation.
"The passage of the Citizenship Amendment Bill marks the victory of narrow-minded and bigoted forces over India's pluralism," said Sonia Gandhi, leader of the main opposition Congress party.
Defending the bill in the upper house, Shah said the new law only sought to help minorities persecuted in Muslim-majority countries contiguous with India.
"Nobody is taking citizenship away from India's Muslims. This is a bill to give citizenship, not take citizenship away," Shah said.
In another move criticized by Muslims as discriminatory, the government scrapped the disputed Kashmir region's autonomy in August.
Last month, the country's supreme court also allowed the construction of a Hindu temple at a religious site in Northern India also claimed by Muslims.
A curfew has been imposed in Guwahati after police clashed with thousands of protesters, beating them back using water cannons and tear gas.
State authorities in Assam blocked mobile internet services in 10 districts, fearing further violence, and some paramilitary troops are being moved from Kashmir to be re-deployed in the northeast, according to a government official.
Protesters, many of them students, remained on the streets late into Wednesday evening, where bonfires were lit, public property vandalized and vehicles set on fire.