India moved thousands of troops into the northeastern state of Assam on Thursday as violent protests erupted against a new law that would make it easier for non-Muslim minorities from some neighbouring countries to seek Indian citizenship.

Prime Minister Narendra Modi's Hindu nationalist government has said the so-called Citizenship Amendment Bill was meant to protect besieged minorities.

Critics say it undermines the country's secular constitution by not offering protection to Muslims while others argue it will open India's northern states to a flood of foreigners.

Resistance to the bill has been the strongest in the tea-growing Assam state, where a movement against illegal immigrants from its neighbouring Bangladesh has simmered for decades.

As India's upper house of parliament passed the bill in the early hours of Thursday, protests took place across India's northeast. In Assam, protesters defied a curfew, torching cars and tyres and chanting anti-Modi slogans.

While the streets of Assam's capital Guwahati were largely calm as troops moved in from neighbouring states, protesters were back on the streets in other parts such as Morigaon, where they burnt tyres.

Mobile internet has been suspended in some parts of Assam for 24 hours until 7 pm Thursday, the government said in an order, adding that social media platforms could potentially be used to "inflame passions and thus exacerbate the law and order situation."