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.
How bad are the protests?
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.
“The central government and I are totally committed to constitutionally safeguard the political, linguistic, cultural and land rights of the Assamese people,” he tweeted.
However, with internet and mobile services shut down, it is unlikely residents would have been able to read the message.
The chief minister of Assam was stranded at the airport for several hours on Wednesday because roads were blocked by protests.
What do protesters want?
They want the bill to be repealed, as they say their ethnic and cultural identity is under threat from illegal migration.
Essentially, they do not want any migrants – regardless of religion – to be allowed into the state.
What is further fuelling passions in Assam, is the fact that two million residents – deemed to be illegal immigrants- were left off a citizens’ register last August.
The National Register of Citizens (NRC) is a list of people who can prove they came to the state by 24 March 1971, a day before neighbouring Bangladesh became an independent country.
In the run-up to its publication, the BJP had supported the NRC, but changed tack days before the final list was published, saying it was error-ridden.
The reason for that was a lot of Bengali Hindus – a strong voter base for the BJP – were left off the list, and would possibly become illegal immigrants.
The CAB is seen as being linked to the register, although it is not the same thing.
It will help protect non-Muslims who are excluded from the register and face the threat of deportation or internment.
Has the bill been challenged?
The Indian Union Muslim League, a political party, has petitioned the country’s top court to declare the bill illegal.
In their petition to the Supreme Court, the Indian Union Muslim League argued that the bill violated articles of equality, fundamental rights and the right to life.