The accuser went to LAPD and filed a report after her alleged Wednesday night run-in with the rapper at a home in the Hollywood Hills ... according to law enforcement sources.

Their interaction began earlier Wednesday. Sources close to the situation tell us they met at a music video set and afterward, Fetty invited the accuser and several other women to a party at an Airbnb. We're told at some point during the evening ... the accuser went into a room with Fetty, and that's when things allegedly turned violent.

TMZ obtained a video shot in that room. When it begins you can hear a woman accusing Fetty of having just put his hands on her, and she dares him to do it again. Fetty then appears to smack the phone out of her hand.

Our sources say there was a witness -- one of the accuser's friends -- in the room at the time. As for what started the argument ... we're told there was an angry discussion about the other women he'd invited back to the house.

Law enforcement sources tell TMZ ... Fetty's now a battery suspect and detectives will attempt to interview him about the allegations.