Is JUSTIN BIEBER losing his mind?  Having trouble dealing with his fame?  Or even having SUBSTANCE ABUSE PROBLEMS?

--People were already wondering that BEFORE Friday morning, when Justin flipped out and tried to attack a photographer in London.

--It happened the morning after Justin collapsed backstage and had to cut a gig short at London's O2 Arena.

--Justin was going from his hotel to his car, when it appears he DELIBERATELY shoved a paparazzi scumbag, then got into the vehicle.

--The guy started shouting "That's assault!" and arguing with Justin's bodyguards.  Then he yelled, quote, "[Effing] little [C-word that rhymes with SPOCK], [eff] off back to America . . . [Effin'] little moron."

--Justin heard the guy, got back out of his car and LUNGED at him.  He yelled, quote, "What the [eff] did you say?  I'll [effing] beat the [eff] out of you!"  Of course, his security team was there to hold him back.

--Justin's bodyguards got him back into the car, while the photographer kept trying to provoke him.  (--Here's video. WARNING!!!  There's some UNCENSORED profanity in this clip.  And here are some pics.)  (The Superficial)

--Justin later Tweeted, quote, "Ahhhhh!  Rough morning.  Trying to feel better for this show tonight but let the paps get the best of me."

--Then he added, quote, "Sometimes when people r shoving cameras in your face all day and yelling the worst thing possible at u...well I'm human.  Rough week."

--"Not gonna let them get the best of me again.  Gonna get focused on this show tonight.  Adrenaline is high now.  Gonna put it on the stage…Only way someone can break u is if u let them."

--Justin did go ahead with his show Friday night in London.  But yesterday his people announced that he's canceling tomorrow night's scheduled gig in Lisbon, Portugal due to, quote, "unforeseen circumstances." 

--There's no word what those circumstances are, but Justin is playing Lisbon tonight . . . and there are rumors that the second show wasn't selling well enough.