11:35 AM PT -- A rep for Serafina tells TMZ the person Bieber allegedly headbutted was not an employee. They also tell us, contrary to law enforcement, no one from the staff called 911. Law enforcement told us the person who called was not the alleged victim but a member of the staff. The restaurant adds, "We always have a pleasant experience with Mr. Bieber whenever he comes to Serafina."

Justin Bieber allegedly headbutted a guy at a pre-Grammy party last weekend, and cops are now investigating the incident as a possible crime.

Law enforcement sources tell us the alleged incident went down at 2 AM Saturday at L.A.'s Italian eatery, Serafina ... where Justin's producer pal, Poo Bear, was hosting a shindig.

Several eyewitnesses tell us Bieber was "play fighting" with Kyle Massey when he realized someone from the restaurant staff was recording the action. We're told Justin asked the guy to stop recording and delete the video. The staffer apparently resisted and some partygoers started recording as well ... that's when the eyewitnesses say Justin lunged at the guy.

We're told a restaurant employee -- not the alleged victim -- called the L.A. County Sheriff's Dept and reported the alleged headbutt. When deputies arrived ... Justin had already left. We got a shot of him on his way out.

We're told the alleged victim doesn't want to press charges -- but the misdemeanor battery investigation is still going forward.

Justin has not been in trouble with the law for more than a year, although a guy filed a police report against Justin Wednesday over a fight after an NBA finals game last June in Cleveland.