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