Justin Bieber's Bodyguard -- Unleashes Fist of Fury on Photog [VIDEO]

Justin Bieber's bodyguard put down an over-agressive photog with one serious haymaker at LAX Monday afternoon ... and the whole melee was caught on tape.

TMZ obtained video of the altercation between the Bieb's protector and the paparazzo -- not one of our guys -- and when the clip begins the bodyguard already has the pap in a headlock.

After several moments of grappling, the photog breaks free to square off with the bodyguard -- BIG mistake ... 'cause that's when the bodyguard uncorks a straight right to the photog's face.

As TMZ first reported, according to Bieber's camp the altercation began when the pap jumped in front of Bieb's SUV and refused to stop snapping pics.

The pap was arrested and booked for felony assault with a deadly weapon. Police believe the fight started when he used his camera to hit the bodyguard.