At least 19 people have been killed in the Indian capital during violent clashes between Hindu and Muslim groups over a new citizenship law, a senior hospital official told Reuters, although there was an eerie calm on Wednesday in the riot-torn areas.

"There are 15 patients in critical condition," the hospital official from the Guru Teg Bahadur Hospital told Reuters, adding a total of 19 people had now died at the facility.

Police used tear gas, smoke grenades and pellets, but have struggled to disperse stone-throwing crowds involved in the violence that spiralled out of control on Monday.

The clashes, which coincided with a visit to India by U.S. President Donald Trump, erupted between thousands demonstrating for and against the new citizenship law introduced by Prime Minister Narendra Modi's Hindu nationalist government.

India's capital has been the epicentre of unrest against the Citizenship Amendment Act, which makes it easier for non-Muslims from some neighbouring Muslim-dominated countries to gain Indian citizenship.

Critics say the law is biased against Muslims and undermines India's secular constitution. The ruling Bharatiya Janata Party has denied it has any bias against India's more than 180 million Muslims.

Reuters witnesses saw mobs wielding sticks, pipes and stones walking down streets in parts of northeast Delhi on Tuesday, amid incidents of arson, looting and stone-throwing. Clouds of black smoke billowed from a tyre market that had been set alight in the area, as fire trucks rushed to control the blaze.

Shots could be heard in the area and many of the wounded had suffered gunshot injuries, hospital officials said.

However, there were no immediate reports of any fighting on Wednesday.

"The situation is relatively better than yesterday in the violence hit areas," Atul Garg, the director of the Delhi fire department told Reuters on Wednesday, adding the fire department had stationed additional vehicles in the area and senior officials are camping there.

"There are no rioters on the streets and our vehicles have been able to reach the area," he said.