Justin Bieber -- Undresses to Mix of Cheers and Boos

Justin Bieber ripped off all his clothes, except for his Calvin Klein boxers, during Tuesday night's "Fashion Rocks" event in Brooklyn -- but flashing his bod and ego nearly backfired.

Biebs was on stage with model Lara Stone to introduce Rita Ora, when he decided to strip down. You can hear boos start to rain down at first, but the more naked he got ... the more the screaming chicks started to win out.

Justin tweeted after the show, "Grandma always said kill 'em with kindness...and then strip on live national tv. Lol."

Thanks for nothing, Granny.