In the country’s northeast, however, even allowing non-Muslims citizenship is opposed by many locals who fear their culture is threatened by an influx of Bengali-speaking Hindus.

Modi, who insists he is not anti-Muslim, said the citizenship law is “1,000 per cent correct” and that Muslims from the three countries are not covered because they have no need of India’s protection.

Rahul Gandhi, former opposition Congress chief, tweeted on Monday that the law and a mooted nationwide register of citizens also seen as anti-Muslim were “weapons of mass polarisation unleashed by fascists”.

At least six people have been killed in the country’s northeast in protests since last week, with up to 100 reported injured in New Delhi.

The unrest began on Sunday evening after a protest march by students from the Jamia Millia Islamia university in Delhi led to violent clashes with police.

Local media reported that more than 70 people including students were injured and at least four buses and several motorcycles were burned.

Police used tear gas and detained 100 students and released them early Monday.

After police were accused of using excessive force to quell the protests in Delhi, clashes began at the prestigious Aligarh Muslim University in northern India.

On Monday fresh protests took place in Delhi, Chennai, Bangalore and Lucknow, where hundreds of students – most of them Muslims, television pictures indicated – tried to storm a police station, hurling volleys of stones at officers cowering behind a wall.

In the east, thousands gathered in Kolkata, capital of West Bengal, for a major demonstration called by the state’s chief minister Mamata Banerjee, a firebrand opponent of Modi.

In recent days empty trains were torched there and on Monday internet access remained suspended.

In Kerala in the south, another state whose government refuses to implement the citizenship law, several hundred people also protested. Kerala’s finance minister Thomas Isaac tweeted: “United action of all secular force is the need of the hour.”

Protests were reported in Mumbai, West Bengal, Aligarh, Hyderabad, Patna and Raipur over the weekend.

Authorities in northern Uttar Pradesh, meanwhile, have cut internet access in western parts of the state following demonstrations in Aligarh, home to a large university and a sizeable Muslim population.

The main epicentre of the protests has been in India’s far-flung northeastern states, long a seething and violent melting pot of ethnic tensions.

There, where protesters are mostly Hindu, late last week four people died from gunshot wounds, one in a fire and a sixth beaten to death.

On Sunday night in Assam state – following days of rioting and clashes with police – around 6,000 people protested on Sunday evening, with no major incidents reported.

Modi blamed the main opposition Congress party and its allies of “stoking fire”, saying those creating violence “can be identified by their clothes” – a comment interpreted by some as referring to Muslims.

The new law is being challenged in the Supreme Court by rights groups and a Muslim political party, arguing that it is against the constitution and India’s cherished secular traditions.

Ashok Swain, a professor at Sweden’s Uppsala University said that the scale of the protests had caught Modi’s government, which is presiding over a serious slowdown in economic growth, off guard.

“The protest is getting international attention and also spreading to different parts of the country. This certainly will add pressure on the regime when the economy has failed,” Swain said.

Additional reporting by Agence France-Presse