In Chabua, a town bordering an Indian air force base, protesters had torched government property, including a post office, a local police official said.
A mob had also set alight the house of local lawmaker Binod Hazarika, from Modi’s Bharatiya Janata Party (BJP).
“They torched it and finished it,” the police official said, declining to be named as he was not authorised to speak to the media. “The situation is very bad here.”
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.
“This is a spontaneous public outburst,” said Nehal Jain, a masters student in communications in Guwahati. “First they tell us there are too many illegal immigrants and we need to get rid of them. Then they bring in this law that would allow citizenship to immigrants,” she said.
More troops have been deployed to Assam to restore peace and mobile internet was suspended in 10 districts, the government said.
A curfew was also to be imposed in parts of the capital city of the neighbouring state of Meghalaya, a government official said, because of fears of the law and order situation deteriorating.
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 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.