A bill that grants Indian citizenship to non-Muslim illegal immigrants has been challenged in the Supreme Court.
The Indian Union Muslim League, a political party, has petitioned the court to declare the bill illegal.
The parliament on Wednesday passed the bill which applies to migrants from Afghanistan, Bangladesh and Pakistan.
Critics say the bill is against Muslims, but the ruling Hindu nationalist Bharatiya Janata Party (BJP) has defended it.
The BJP says the Citizenship Amendment Bill (CAB) will give sanctuary to people fleeing religious persecution.
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.
It was passed in the upper house of parliament by 125 votes to 105 on Wednesday. It had already cleared the lower house. It will become a law once the president signs it, which is a formality at this stage.
The CAB has also triggered huge protests in the north-eastern state of Assam, forcing authorities to declare a curfew and shut down internet services.
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 still shut down, correspondents say it is unlikely residents would have been able to read his tweets.
The situation in Assam remains tense, as people defied curfew to protest in the state capital, Guwahati, on Thursday morning. Train services are suspended and some airlines have started offering rescheduling or cancellation fee waivers.
Protests have also taken place in the north-eastern state of Tripura which borders Bangladesh.
Why are there protests in the north-east?
People in Assam and Tripura fear that they will be "overrun" by illegal non-Muslim migrants from neighbouring Bangladesh.
The army has been sent into the state of Tripura and thousands of troops have been deployed to Assam.
The chief minister of Assam was stranded at the airport for several hours on Wednesday because roads were blocked by protests.
A curfew was declared in the state capital and internet and mobile services were blocked in a number of districts to try to prevent further violence.
Violent anti-migrant protests in the same region meant that the bill could not be passed in the upper house when it was tabled ahead of general elections earlier this year.
The protesters are particularly vocal in Assam, which in August saw two million residents left off a citizens' register.
Illegal migration from Bangladesh has long been a concern in the state.
The CAB is seen as being linked to the register, although it is not the same thing.
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 also left out of the list, and would possibly become illegal immigrants.
The CAB will help protect non-Muslims who are excluded from the register and face the threat of deportation or internment.
What are critics saying?
Those protesting against the bill say it is part of the BJP's agenda to marginalise Muslims and violates India's secular principles.
"Muslims are already being persecuted. This will make them more vulnerable," a participant at a protest against the bill in Delhi told the BBC.
More than 700 eminent Indian personalities, including jurists, lawyers, academics and actors, have signed a statement "categorically" condemning the bill.
It says that the government seems "intent on causing huge upheavals within Indian society".
Many others have questioned why it only refers to non-Muslim migrants from Pakistan, Bangladesh and Afghanistan, when minorities from other neighbouring countries were also being persecuted.
Tamil film superstar-turned-politician Kamal Haasan asked why the same courtesy was not being extended to Sri Lankan migrants from minority communities.
In parliament as well, a number of opposition parties and politicians raised similar concerns.
Prominent Muslim MP Asaddudin Owaisi said it was "worse than Hitler's laws and a conspiracy to make Muslims stateless".
A senior leader of the main opposition Congress party, Rahul Gandhi, said anyone supporting the bill was "destroying India's foundation". And a leading regional politician Akhilesh Yadav called it a "divisive plot to divert attention" from the failures of the government.
But BJP leaders, including Home Minister Amit Shah, say the bill is not against Muslims.
"The Muslims of this country don't have to worry about anything. But should the Muslims of Pakistan be made citizens? Should Muslims from Bangladesh and Afghanistan and the rest of the world also be given citizenship? The country cannot run like this. The citizenship will be given only to persecuted religious minorities only from these three countries," Mr Shah said in parliament.