Violent clashes erupted in Delhi amid allegations a new citizenship bill discriminates against Muslims and undermines the secular foundations of India, with protests over the legislation spreading to other regions and prompting Japan’s prime minister to cancel a visit to the country.

Thousands took to the streets of Assam’s capital Guwahati for the third day, following the death of two protesters who were caught in police fire on Thursday. The north-eastern state has been the epicentre of the protests against the citizenship amendment bill (CAB).

A 48-hour block on both mobile and broadband internet was implemented, while schools were closed and roads blocked off, as demonstrators set fire to cars and tyres and a 10pm curfew was imposed on the city.

The controversial bill, which was signed into law at midnight on Thursday, has stirred up opposition in Assam and across the country. Under the new legislation, tens of thousands of Hindu, Christian, Jain, Buddhist and Sikh migrants from Bangladesh, Pakistan and Afghanistan will be allowed to claim Indian citizenship. The same will not apply for Muslims.

Critics have accused the BJP government of passing a law which openly discriminates against Muslims and makes them second-class citizens in India. Many have argued it violates India’s secular constitution and several petitions were filed in the supreme court on Friday challenging its constitutional validity, including by Congress MP Jairam Ramesh and Trinamool Congress MP Mahua Moitra.

A police officer fires a teargas shell during the protest. Photograph: Adnan Abidi/Reuters

A Muslim police constable in Guwahati, who didn’t want to be identified because of fear of a reprisal from the police department, said: “Originally I am from Uttar Pradesh but I have been working here [Assam] for the past 30 years in Assam police. But in the last few years it’s been very hard to live peacefully as a Muslim. BJP has made this country a filthy place. They want to turn this into a Hindu Rashtra [Hindu state]. Where will we go? Sometimes even I am scared even though I am in police.”

The students of Assam have been among those leading the protests, including a mass hunger strike that began on Friday. Masters student Simanta Thakuria, who was among the organisers, said: “This bill has to go back at any cost. We will not rest until we get this done. Thousands of people have joined us, as you can see and it’s completely non-violent. We don’t support any form of violence. This is our state, our land. Not only in Guwahati but people are protesting everywhere in Assam.”

In Delhi, protests turned violent after hundreds of students demonstrating at Jamia Millia Islamia university were fired at with teargas by police, who tried to push the crowds back with batons. Several students were taken to hospital with injuries while over 50 protesters were detained.

Protesters set fire to placards in Amritsar and other rallies were held in Kolkata, Kerala and Gujarat, the home state of the prime minister, Narendra Modi.

There was also unrest in West Bengal, with several trains vandalised. The state is due to be the first where a new national register of citizens (NRC) is rolled out, which will require citizens to provide documents to prove they are not illegal immigrants. Following the passing of CAB into law, it is feared that only Muslims will be targeted by the exercise and be detained in their thousands. However West Bengal chief minister, Mamata Banerjee, has vowed that she will not allow CAB to be rolled out in Assam and announced state-wide protests. Over 20 activists were also arrested in Mumbai for protesting the bill.

The unrest led the Japanese prime minister, Shinzō Abe, to cancel a planned visit to India from Sunday for a summit with Modi.

On Friday, the United Nations human rights office also voiced concern that the new citizenship law was “fundamentally discriminatory in nature” by excluding Muslims and called for it to be reviewed.