Police have used tear gas to disperse large crowds protesting a new citizenship law in India's capital New Delhi just a day before US President Donald Trump begins a two-day visit of the country.

In the latest eruption of violence at protests on Sunday, hundreds of people supporting the new law clashed with those opposing it, with the two groups pelting each other with stones in the Maujpur area in the northeastern part of the city, according to television footage.

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On Sunday, a separate protest also erupted in the northern Indian city of Aligarh, where protesters threw stones at the police, state administration official Chandra Bhushan Singh said.

India's Citizenship Amendment Act, which eases the path for non-Muslims from neighbouring Muslim-majority nations to gain citizenship, has triggered weeks of sometimes violent protests against Prime Minister Narendra Modi's government.

The law is seen by opponents as discriminating against Muslims and has deepened concerns that Modi's administration is undermining India's secular traditions.

Trump is expected to raise the issue of religious freedom in the country with Modi. The US president left for India on Sunday, predicting that "many millions" will greet him on a lightning visit with Modi.

"I look forward to being with the people of India," Trump said as he left the White House on his first trip to India.

"We're going to have many millions and millions of people," he said. "I hear it's going to be a big event, some people say the biggest event they ever had in India. That's what the prime minister told me."

Ahead of the India trip, which will see him and his wife Melania visit the Taj Mahal and attend a rally with Modi in the world's largest cricket stadium, he has repeatedly mused on the "millions" he expects to turn out.

Enormous crowds are guaranteed during the one-night trip, given Modi's own prowess at staging mega-rallies.