(TMZ) - 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 told tmz 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 told TMZ Bieber was "play fighting" with Kyle Massey when he realized someone from the restaurant staff was recording the action. TMZ reported that 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.

TMZ was 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. TMZ got a shot of him on his way out.

TMZ was 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.

TMZ reached out to Justin's camp ... so far no word back.

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