Justin Bieber -- 'Bodyguard' Accused of Threatening Shopper

A member of Justin Bieber's crew allegedly threatened a man who snapped a pic of JB Sunday night, and now police are investigating ... TMZ has learned.

According to law enforcement sources ... Justin was shopping at a Best Buy in the San Fernando Valley when the adult male -- not a crazed tweener fan -- approached him and took the photo.

We're told the man claims someone in Justin's entourage -- someone he thinks is a bodyguard -- immediately, and angrily, told him to delete the photo ... which he did.

The photo opportunist -- who we're told is not a paparazzo -- felt threatened enough after the encounter that he called police ... but when LAPD officers arrived Justin and his posse were long gone.

We're told police took a report from the alleged victim, and plan to investigate his claim.