(TMZ) -- Justin Bieber went from funny to fuming in a hot second Saturday morning ... so says the guy who claims he got clocked.

TMZ broke the story ... Justin's a suspect in an alleged battery that went down at a pre-Grammy party at Serafina restaurant in L.A., after Dean Parker fired up his camera and recorded Justin fake fighting with Kyle Massey.

The video shows a playful Justin who turns angry after someone screams for people to stop shooting video. Parker says Justin lunged for his camera, striking him in the face ... although you don't see the strike on the video. Cops told TMZ it was reported as a headbutt.

It's interesting ... TMZ got this pic of Justin's freshly cut hand as he left the restaurant. See the photo and video here

Photo: Getty Images