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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.