Three men have been arrested in Sweden on suspicion of taking part in a rape that was streamed live on a closed Facebook group.

Police spokeswoman Lisa Sannervik said the investigation into a serious sexual offence is in a preliminary phase.

A teenager and two men in their 20s were arrested in the city of Uppsala, north of Stockholm, after police received calls about the ongoing streaming from users.

Deputy chief prosecutor Magnus Berggren told Sweden's TV4 channel investigators do not have the footage showing the alleged assault, and urged anyone with access to it to contact authorities.

Police in Uppsala were contacted in the morning by a woman who said she had seen a gang rape broadcast in a closed group on the site.

"You have been raped," one of the men said at the end of the video and then laughed

"You have been raped," one of the men said at the end of the video and then laughed, according to the viewer.

Speaking to Swedish tabloid Expressen, a woman said she saw one of the men tear the woman's clothes off and lie on top of her. She also said one of the men had a gun.

Facebook Live shows a count of other people simultaneously watching the broadcast and Ms Lungren said she could see 60 other people viewing.

"Three against one hahaha," one of the viewers wrote in the comments section underneath the video, she said.

Online witnesses told Swedish media they had also seen a second video where the same woman then denied being raped, but there was speculation as to whether or not she had done so under her own free will.

According to Sweden's state broadcaster SVT, the men were still filming the follow-up video when the police arrived.

Facebook Live allows anyone to broadcast a video directly from their smartphone. But while traditional TV broadcasters are subject to regulations, internet streaming services do not face the same restrictions and enforce their own terms of service.