The army has been called into north-eastern India, after thousands of people defied curfews to protest against a new citizenship bill.
The Citizenship Amendment Bill (CAB) offers amnesty to non-Muslim illegal immigrants from three countries.
Critics say the bill discriminates against Muslims - but in the north-east, protesters claim they will be "overrun" by Hindus from Bangladesh.
Indian Prime Minister Narendra Modi has appealed for calm.
Officials said 20-30 people were injured in the demonstrations, and air and railway services have been severely impacted.
The bill - which applies to people from Afghanistan, Bangladesh and Pakistan - was passed in the upper house of parliament on Wednesday night.
It is yet to be ratified by the president, but that is merely a formality.
The ruling Hindu nationalist Bharatiya Janata Party says the CAB will give sanctuary to people fleeing religious persecution.
Illegal migration from Bangladesh has long been a concern in the north-east.
Violent protests intensified on Thursday, and have been particularly bad in the states of Assam and Tripura, which border Bangladesh.
The army has deployed thousands of personnel, as protesters defy curfew orders and spill into the streets.
The protesters blocked roads and set vehicles on fire. There are reports that at least two railway stations have been burned down.
Railway services are suspended and some airlines have started offering rescheduling or cancellation fee waivers.
The AFP news agency reported that police fired blanks into the air in a bid to disperse crowds. They have also used tear gas shells.
Prime Minister Narendra Modi sought to reassure people in Assam, telling them they had "nothing to worry" about.
BDST: 1746 HRS, DEC 12, 2019