Trey Songz is being accused of pummeling a cameraman who just got out of the hospital with serious injuries, and police are now investigating.

Isaa Mansoor tells TMZ Hip Hip he went to the Ivy in Huntington, NY, early Sunday morning to shoot video for the owner. Mansoor says he witnessed Trey arrive with an entourage.

Isaa says the owner told him to shoot the event as Trey performed and circulated. Isaa says Trey was aware the owner had asked photogs to get video.

At the end of the event, the owner asked Isaa to get one last pic of Trey next to the Ivy’s logo … in the front of the building. Isaa says he began filming and that’s when Trey attacked him, punching him in the side of the head, throwing him against a wall and smashing two of his cameras.

Isaa says the owner then got involved, telling Trey, “That’s my guy.” Trey responded, “I don’t give a f***,” and got into the owner’s face with a clenched fist.

Isaa filed a police report and cops tell us they have launched an investigation.

We reached out to Trey’s rep … so far, no word back.

Mansoor says he fled the area and went to his friend’s car. Trey fled the scene, and Mansoor called the police to report the assault.

Mansoor later went to the hospital, where he underwent a CT scan and X-rays.

There was heavy damage … he says he was diagnosed with a migraine, ankle pain, and a concussion, and was given pills to relieve the pain.