Prime Minister Narendra Modi's Hindu nationalist government has said the Citizenship Amendment Bill, approved by parliament on Wednesday, was meant to protect minorities from Bangladesh, Pakistan and Afghanistan.
Protesters in the northeastern state of Assam, which shares a border with Bangladesh, say the measure will open the region to a flood of foreigners. Others said the bigger problem with the new law was that it undermined India's secular constitution by not offering protection to Muslims.
Police fired tear gas in Assam's main city of Guwahati to break up small groups of people who were demonstrating in the streets, defying a curfew imposed on Wednesday.
A movement against illegal migrants has simmered in tea-growing Assam state for decades.
The citizenship amendment law grants Indian nationality to Buddhists, Christians, Hindus, Jains, Parsis and Sikhs who fled Afghanistan, Bangladesh and Pakistan before 2015.
Protesters vandalised four railway stations in Assam and tried to set fire to them, a railway spokesman said. Train services were suspended, stranding scores of passengers. IndiGo said it had cancelled flights because of the unrest in Assam.
SUMMIT
The turmoil in Assam comes days ahead of a summit Modi plans to host for Japanese Prime Minister Shinzo Abe there as part of his campaign to move high-profile diplomatic events outside of Delhi to showcase India's diversity.
Modi urged calm and said the people of Assam had nothing to fear. "I want to assure them - no one can take away your rights, unique identity and beautiful culture. It will continue to flourish and grow," he tweeted.
The new law is also raising concerns that Modi's government is pushing a Hindu-first identity for India and fanning fears for the future of Muslims, the biggest minority group.
The Indian Express said the law, which requires presidential assent, unfairly targets India's 170 million Muslims.
"It is a political signal of a terrible narrowing, a chilling exclusion, directed at India’s own largest minority. India is to be redefined as the natural home of Hindus, it says to India’s Muslims. And that they must, therefore, be content with a less natural citizenship."
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 law, by contrast, have a clear path to citizenship.