The United Nations human rights office voiced concern on Friday that India's new citizenship law is "fundamentally discriminatory in nature" by excluding Muslims and called for it to be reviewed.

Violent clashes erupted in Delhi between police and thousands of university students on Friday protesting the enactment of the contentious new law. Muslims also staged peaceful demonstrations in cities such as Ahmedabad and Amritsar. Previous days have seen violent protests in the north-eastern state of Assam, where at least two people were killed in police firing on Thursday.

The protests forced Japanese Prime Minister Shinzo Abe to cancel a trip to India. He was scheduled to arrive on Sunday for a three-day visit during which he was to hold a meeting with Prime Minister Narendra Modi in Guwahati, Assam's state capital.

An Indian foreign ministry spokesman did not give a reason when announcing the postponement of Mr Abe's trip on Friday, but Japan's Kyodo news agency reported it was cancelled because of the worsening security situation, citing the country's chief cabinet secretary.

Burnt vehicles block a road in Guwahati, capital of India's Assam state, on December 13, 2019, a day after protests against the government's Citizenship Amendment Bill broke out across the north-eastern state. AFP

Mr Modi's Hindu nationalist government has said the Citizenship Amendment Bill, approved by parliament on Wednesday, was meant to protect Hindus, Buddhists, Sikhs, Parsis, Jains and Christians who fled persecution in Bangladesh, Pakistan and Afghanistan.

But the law does not extend the same protection to Muslim migrants, the UN rights office said, thereby undermining equality before the law enshrined in India's constitution.

"We are concerned that India’s new Citizenship (Amendment) Act 2019 is fundamentally discriminatory in nature," UN human rights spokesman Jeremy Laurence said in Geneva.

"We understand the new law will be reviewed by the Supreme Court of India and hope it will consider carefully the compatibility of the law with India's international human rights obligations," he said.

Indian President Ram Nath Kovind signed the bill into law late on Thursday despite the protests against it.

Two people were killed and 11 injured in Assam on Thursday when police opened fire on mobs torching buildings and attacking railway stations.

A movement against immigrants from neighbouring Bangladesh has raged in Assam for decades, but this is the first time since 1979 that the state has witnessed protests of this scale, according to Sanjoy Hazarika, professor at the Centre for North East Studies and Policy Research at Jamia Millia Islamia University. Mass demonstrations against illegal migrants lasted for six years in the early 1980s and left 855 people dead and disrupted the economy before the state and federal governments signed an accord to end the movement in 1985.

“Nobody wants a return to that era,” Mr Hazarika told Bloomberg News. “This is much bigger than anything we’ve seen.”

The protesters in Assam say granting Indian nationality to more people will further strain the resources of the state and lead to the marginalisation of indigenous communities.

Japan has stepped up infrastructure development work in the state in recent years which the two sides were expected to highlight during the summit. Mr Abe had also planned to visit a memorial in the nearby state of Manipur where Japanese soldiers were killed during the Second World War.

Critics of Mr Modi's Hindu nationalist government say the bigger problem with the new law is that it is the first time India is using religion as a criterion for granting citizenship and that it excludes Muslims from its ambit.

The Indian Union Muslim League party has petitioned the Supreme Court saying the law was in conflict with the secular principles of India's constitution that guaranteed equality to all without any regard to religion. No date has yet been set for the hearings.

The party said the law is "prima facie communal" and questioned the exclusion of minorities such as Rohingya Muslims who were just as persecuted as other faiths listed in the law.

Adding to the fear and anxiety generated by the new law is Mr Modi’s government's pledge to implement a citizenship drive nationwide to weed out undocumented migrants. Assam was the first state to compile a National Citizenship Register, an lengthy process that ended in August with about 1.9 million people at the risk of being stateless.