Indian protesters angry over a new federal law that would make it easier for non-Muslim minorities from some neighbouring countries to seek Indian citizenship have attacked train stations and blocked highways in the country's north-east, Indian authorities have said.

Key points: The citizenship amendment law grants Indian nationality to non-Muslims who fled Afghanistan, Bangladesh and Pakistan

The citizenship amendment law grants Indian nationality to non-Muslims who fled Afghanistan, Bangladesh and Pakistan Protesters say the measure will open the region to a flood of foreigners

Protesters say the measure will open the region to a flood of foreigners An upcoming citizenship register could potentially render some Muslims stateless

Prime Minister Narendra Modi's Hindu nationalist government said the Citizenship Amendment Bill, approved by parliament on Wednesday, was meant to protect minorities from Bangladesh, Pakistan and Afghanistan.

Protesters in the north-eastern state of Assam, which shares a border with Bangladesh, said the measure will open the region to a flood of foreigners.

Protesters set fire to tyres and built burning barricades to block roads. ( AP: Anupam Nath )

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.

Others set fire to tyres and built burning barricades to block roads.

"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.

Protesters say the measure will open the region to a flood of foreigners. ( AP: Anupam Nath )

'A chilling exclusion' of 170 million Muslims

The new law is 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.

The new law will be followed by a citizenship register that could potentially render some Muslims stateless. ( AP: Anupam Nath )

"It is a political signal of a terrible narrowing, a chilling exclusion, directed at India's own largest minority," the Express said.

"India is to be redefined as the natural home of Hindus, it says to India's Muslims."

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.

Transport shut down amid protests

More troops have been deployed to Assam to restore peace. ( AP: Anupam Nath )

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.

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," he tweeted.

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More troops have been deployed to Assam to restore peace and mobile internet was suspended in 10 districts, the Government said.

ABC/Reuters