Justin Bieber has sparked fears he is on the verge of a meltdown after launching himself at a photographer in London on Friday (08Mar13) and challenging him to a fight.

The pop superstar left his hotel in the British capital on Friday afternoon and dashed to a waiting car, knocking into a snapper outside as he jumped into the vehicle.

The photographer was heard complaining about being shoved out of the way by the star, when Bieber opened the car door and launched himself at the man, yelling, "What the f**k did you say? What did you say? I'm gonna f**king beat the f**k out of you."

Bieber was held back by members of his entourage as he attempted to swing punches at the snapper, before being bundled back into the car.

He subsequently addressed the drama in a series of posts on his Twitter.com page, writing, "Ahhhhh! Rough morning. Trying to feel better for this show tonight but let the paps (sic) get the best of me... Sometimes when people r (are) shoving cameras in your face all day and yelling the worst thing possible at u (sic)... 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."

Bieber is preparing for his final show at London's O2 Arena on Friday night after collapsing backstage during Thursday's performance. He was subsequently hospitalised, but doctors discharged him and he was deemed well enough to perform.

The latest incident comes after a tumultuous 10 days in London for the star, which included a dispute with nightclub bouncers on his birthday and coming under fire from fans for starting his Monday night (04Mar13) show late.