A video showing two women being paraded naked by a mob in the violence-hit northeastern state of Manipur has sparked outrage in India.
The police say they have opened a case of gang rape and have arrested a man, adding that others will be held soon.
This is also likely to dominate discussions in the parliament's monsoon session, which begins on Thursday.
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 began.
The Supreme Court also expressed its concern over the assault. Chief Justice of India DY Chandrachud said the court was "really disturbed over the video" and asked the government to take action.
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 have died and tens of thousands have been displaced since ethnic clashes started in May in Manipur.
Clashes broke out between the Meitei and Kuki communities after the Meiteis - the state's main ethnic group - demanded tribal status which gives access to benefits such as forest land and government job and education quotas.
Some 60,000 have become refugees in their own land. Opposition politicians have criticised PM Modi for not visiting the state or speaking about the violence in Manipur so far.
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 a Kuki mother and daughter duo, who were stripped naked and paraded while being groped and molested by a mob of men in Manipur, is the latest example of that.
The video 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 after the outrage in the case that happened 78 days ago and was reported to the police more than two months back doesn't inspire much confidence in the administration - more so since many of the men are clearly identifiable in the footage.
But the massive outrage that has 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.
To restore some sort of confidence in the people of violence-torn Manipur, especially the minority Kuki community, the authorities must now act swiftly against the perpetrators of the horrific assault and bring justice to the women. 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 against "unknown armed miscreants" at Thoubal district.
The police added that they were making an "all out effort to arrest the culprits at the earliest".
The incident has sparked strong reactions from politicians across the spectrum.
Federal minister Smriti Irani, called the incident "condemnable and downright inhuman". She also said that she had spoken to Manipur Chief Minister N Biren Singh, who informed her that investigations were underway and that "no effort will be spared to bring perpetrators to justice".
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", adding that women and children face the maximum brunt of violence in society.
She asked why the federal government and the prime minister "were sitting blindly on the violent actions in Manipur".
"Do such images and violent incidents not disturb them?" she tweeted.
Delhi Chief Minister Arvind Kejriwal also condemned the incident. "This kind of heinous act cannot be tolerated in the Indian society," he said.
"The situation in Manipur is becoming very worrying. I appeal to the prime minister to pay attention to the situation in Manipur. Please take strict action against the culprits seen in the video of this incident. There should be no place for people of such criminal nature in India," he tweeted.
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