(TMZ) - Justin Bieber's feeling better - in  fact, he's feeling good enough to FIGHT ... 'cause he just went after a photog  in London... and screamed, "I'LL F***ING BEAT THE F*** OUT OF  YOU."

Bieber - flanked by bodyguards - was leaving the  Central London hotel ... and in the mad scramble to his SUV, he made contact  with a waiting photog.  The photog - clearly upset by the physical  contact - immediately claimed he was assaulted... and began to cuss out Bieber,  who was already inside the waiting SUV.

But Bieber clearly  heard the photog's insults ("f*** off back to America", "f***ing little moron")  ... 'cause the singer opened his SUV door and sprung out of the car ... trying  to attack the photog.

While Bieber's bodyguard held  him back, the singer screamed ... "What the f*** you  say?"

The photog shot back - "You heard what I said... you  heard what I f***ing said, mate."

Bieber replied, "I'll  f***ing beat the f*** out of you."

Justin tried to swing at  the photog, but his bouncers kept the singer at bay... and eventually pushed him  back inside the SUV.  The car left the scene without further  incident.

CLICK HERE TO SEE THE VIDEO - BEWARE EXPLICIT  LANGUAGE

PHOTOS: Splash News

Bieber just tweeted about the incident ... saying, "Ahhhhh!  Rough morning. Trying to feel better for this show tonight but let the paps get  the best of me."