A video showing two women being paraded naked by a mob in the north-eastern state of Manipur, hit by violent ethnic clashes, has sparked outrage in India.
The police say they have opened a case of gang rape and arrested a man, adding that others will be held soon.
On Thursday, parliament's session in Delhi was disrupted as lawmakers demanded a debate on the issue.
Prime Minister Narendra Modi also said the incident had "shamed India" and that "no guilty will be spared".
"I assure the nation, the law will take its course with all its might. What happened with the daughters of Manipur can never be forgiven," he said, finally breaking his silence on Manipur more than two months after violence erupted.
Chief Justice of India DY Chandrachud also expressed concern over the assault, saying the Supreme Court was "deeply disturbed over the video". Telling the government to inform the court about the steps being taken against the accused, the chief justice said "we will take action if you don't".
Deadly violence has plunged Manipur, a scenic Indian state bordering Myanmar, into turmoil for more than two months.
Clashes between members of the majority Meitei and the Kuki tribal communities have resulted in their complete segregation. At least 130 people have died and 60,000 have been displaced.
The two women, who are Kukis, were assaulted by men of the Meitei group.
Police say the assault on the women took place on 4 May but it made national headlines on Thursday after the video started going viral on social media. The federal government has asked all social media companies to delete the video from their platforms.
The horrific video of the two women was widely shared on social media on Wednesday. It shows them being dragged and groped by a mob of men who then push them into a field.
The Indigenous Tribal Leaders' Forum (ITLF) said in a statement that the atrocities had been committed in a village in Kangpokpi district against women from the Kuki-Zo tribal community. It also alleged that the women had been gang raped.
"The gang rape of the women happened after the village was burnt down and two men - one middle-aged and another a teenager - were beaten to death by the mob," the ITLF said.
But the police complaint filed by a relative of one of the women said only one of them was gang raped. It added that a third woman had been forced to strip but she is not seen in the video.
Police said that the incident took place on 4 May and that a case of abduction and gang rape and murder had been registered in Thoubal district.
The assault has been condemned by politicians across the spectrum.
Federal minister Smriti Irani, called it "downright inhuman".
Several opposition leaders also criticised the Bharatiya Janata Party government for not doing enough to quell the violence in the state.
Congress party leader Priyanka Gandhi Vadhra said that the "images of sexual violence against women from Manipur are heart wrenching".
Delhi Chief Minister Arvind Kejriwal weighed in too. "This kind of heinous act cannot be tolerated in the Indian society," he said.
Source: BBC