Justin Bieber -- Standoff with Security After Mayweather Fight ... Don't Make Me Cuff You!! (VIDEO)

Justin Bieber had to square off with a ringside security guard just so he could get to his pal Floyd Mayweather Jr. to celebrate his victory.

The video is kinda bizarre ... since everyone knows Justin and Floyd are as tight as can be -- but one security guard apparently missed Bieber walking ... INTO THE RING with the champ.

Witnesses tell us Justin was actually in the ring until police politely asked him to step out during post-fight ceremonies. The video begins as Bieber tries to get back in, only to have the guard stand his ground and bark ..."You wanna be escorted out in handcuffs, young man?"