Photo taken on Dec. 15, 2019 shows passenger buses in flames in New Delhi, India. Protesters in Indian capital Sunday set ablaze several vehicles including three buses during their protest over new citizenship law in country, officials said. (Str/Xinhua)

Fresh protests rocked India on Monday as anger grew over new citizenship legislation slammed as anti-Muslim, after six people died in the northeast and up to 200 were injured in New Delhi.

The law fast-tracks citizenship for non-Muslims from three neighboring countries. Critics say it is part of Indian Prime Minister Narendra Modi's Hindu nationalist agenda to marginalize the 200 million strong Islamic minority.

Modi on Monday denied this, tweeting that the new law "does not affect any citizen of India of any religion," while accusing "vested interest groups" of stoking the "deeply distressing" unrest.

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

The UN Human Rights Office said last week it was concerned the law "would appear to undermine the commitment to equality before the law enshrined in India's constitution," while Washington and the European Union have also expressed unease.

On Monday fresh protests took place including 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.

The northeast, where even allowing non-Muslims citizenship is opposed by many locals and which in recent days has been the epicentre of protests with six people dead, also saw fresh demonstrations.

In the east in Kolkata, capital of West Bengal, thousands took part in a march led by state premier Mamata Banerjee, a firebrand opponent of Modi. Protestors set fire to tires on train lines.

Students gathered again at Delhi's Jamia Millia Islamia university on Monday, a day after police with batons fired tear gas and charged protesting students before storming the building.

The university's vice chancellor ­Najma Akhtar said on Monday that 200 people were injured but police put the number at 39 students hurt with 30 ­officers also injured, one of them critically.

Police spokesman MS Randhawa said that four buses, 100 private vehicles and 10 police bikes were damaged, and that officers exercised "maximum restraint, minimum force" despite being "provoked."

He denied some media reports that police opened fire.