A video showing two women being paraded naked by a mob in the violence-hit north-eastern state of Manipur 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".
Warning: This article contains details some readers may find distressing.
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.
At least 130 people have died and 60,000 have been displaced since ethnic clashes started between the Meitei and Kuki communities in May in Manipur.
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.
'This shouldn't be happening in modern India'
By Geeta Pandey, BBC News, Delhi
It's well known that women's bodies often become a battleground during riots and conflicts, and rape and sexual assault are used as instruments of violence to punish them.
The sexual assault of the Kuki mother and daughter in Manipur is the latest example. The footage showing the women weeping, wincing in pain and begging their attackers to show some mercy is disturbing to watch.
The fact that the first arrest has been made only now, more than two months after the attack was reported to police, does not inspire confidence in the authorities - all the more so since many of the men are clearly identifiable in the footage.
But the outrage that followed the video's emergence in India has put the spotlight on the horrific crime. It has also raised questions about the failure of the state in comforting the survivors - and finally forced Mr Modi to make a statement on the ethnic violence that is tearing Manipur apart.
To restore some sort of confidence in Manipur, especially among the minority Kuki community, the authorities are now under pressure to act swiftly against the perpetrators and bring justice to the women. People across the country feel this should not be happening in modern India.
"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.
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.