Sporadic violence hit parts of Delhi overnight as gangs roamed streets littered with the debris of days of communal riots that have killed 32 people, police said on Thursday.

Thousands of riot police and paramilitaries patrolled the affected northeast fringes of the Indian capital of 20 million people, preventing any major eruptions.

Protests against a contentious citizenship law began on a smaller scale on Sunday but escalated on Monday — as US President Donald Trump started his two-day trip to India — and Tuesday into running battles between Hindus and Muslims in New Delhi’s north-east, where rioters armed with stones, swords and even guns were out in force.

Homes, shops, two mosques, two schools, a tyre market and a fuel station were torched.

More than 200 people were also injured.

Meanwhile, the US Commission on International Religious Freedom, which advises the US government but does not set policy, voiced “grave concern” about the violence which broke out as President Donald Trump was visiting New Delhi.

“One of the essential duties of any responsible government is to provide protection and physical security for its citizens, regardless of faith,” said chairman Tony Perkins, a conservative Christian close to the Trump administration.

“We urge the Indian government to take serious efforts to protect Muslims and others targeted by mob violence,” he said in a statement.

A commissioner appointed by Democratic House Speaker Nancy Pelosi, Anurima Bhargava, voiced alarm at reports that Delhi police “have not intervened in violent attacks against Muslims.”

“The brutal and unchecked violence growing across Delhi cannot continue,” she said.

“The Indian government must take swift action to ensure the safety of all of its citizens,” she added.

The criticism stands in contrast to the reticence of the Trump administration.

Trump, asked at a news conference in New Delhi about the violence, said the issue was “up to India” and praised Prime Minister Narendra Modi's “incredible” statements on religious freedom.

The two leaders of the US Senate's pro-India caucus, Republican John Cornyn and Democrat Mark Warner, in a joint statement voiced support for the close US ties represented by Trump's visit but added: “At the same time, we are alarmed by the recent violence in New Delhi.”

Representative Pramila Jayapal, an Indian-born left-leaning Democrat who has been outspoken in her criticism of Modi, called the developments “horrifying.”

“Democracies should not tolerate division and discrimination, or promote laws that undermine religious freedom. The world is watching,” she wrote on Twitter.

Modi has called for calm, although witnesses said police did little to stop Hindu mobs.

His government has previously vowed to weed out “infiltrators” from India, with Home Minister Amit Shah likening undocumented immigrants to “termites.”

The government says the citizenship law does not target minorities but instead ensures protection for non-Muslims persecuted in neighbouring countries.

The Indian foreign ministry previously reprimanded the US Commission for International Religious Freedom for denouncing the citizenship law.

The commission also plans a public hearing next week on how citizenship laws, including in India and Myanmar, are used to target religious minorities.