Three men have been arrested on suspicion of raping a woman and reportedly live-streaming the footage on Facebook.
A woman contacted police in Uppsala, Sweden on Sunday morning to say she had seen a gang rape broadcast live in a closed group on the site, the BBC reported.
The attack is thought to have taken place in an apartment in Uppsala while the victim was "close to unconscious", reported The Sun.
The online witness reported one of the men as having said "You have been raped," and then laughing at the end of the video.
Josefine Lundgren, 21, who called the police when she saw the footage, said she saw one of the men tear the woman's clothes off and lie on top of her. Speaking to Swedish tabloid Expressen, she also said one of the men had a gun.
Facebook Live showed that there were 60 other people simultaneously watching the broadcast, she said. One viewer wrote a comment beneath the video which read: "Three against one hahaha," Lundgren added.
Others who saw the footage told Swedish media that they had seen a second video in which the same woman denied being raped. However there was speculation that she may not have done so of her own accord.
The men were still filming the follow-up video when the police arrived, according to Sweden's state broadcaster SVT.
Another viewer who saw the footage told Expressen they "thought it was a poorly orchestrated joke".
Regional police confirmed that they and "many" others had seen the video. They have begun an investigation into the incident and forensic scientists are examining the apartment, The Sun reported.
A Facebook spokesperson told Refinery29: “This is a hideous crime and we do not tolerate this kind of content on Facebook. We take our responsibility to keep people safe on Facebook very seriously. Our community standards regulate what kinds of content can be shared on Facebook. Our teams work around the clock to review content that is being reported by users and we have systems in place to ensure that time sensitive content is dealt with quickly."
If someone violates the community standards while using Facebook Live, they said, "we want to interrupt these streams as quickly as possible when they're reported to us. So we've given people a way to report violations during a live broadcast."
The spokesperson added: "We support local law enforcement who make data requests related to criminal investigations, particularly when it comes to the safety of young people. We respond to valid requests relating to criminal cases.”