At least 21 people were killed and 189 injured in three days of clashes in New Delhi that coincided with US President Donald Trump's first state visit to India, with the death toll expected to rise as hospitals continue to take in the wounded, authorities said on Wednesday.

Violence broke out on Sunday after Hindu mobs attacked Muslims protesting against a new citizenship law that critics say violates the secular ethos of the South Asian country.

The violence-affected areas were tense on Wednesday with most shops and schools closed. The city government, led by Chief Minister Arvind Kejriwal called for the army to be deployed as police were accused of partisanship.

The Hindu nationalist government has banned public assembly in the affected areas to prevent violence.

On Wednesday, Prime Minister Narendra Modi called for calm but he has been criticised for not acting on time and hosting a lavish reception for Trump while Delhi burned for three days.