"Ahhhhh! Rough morning. Trying to feel better for this show tonight but let the paps get the best of me," the singer posted on Twitter soon after the altercation with the photographer, which took place as he got into a car earlier Friday. The scuffle was captured on video by Channel 5 News and widely broadcast by Britain's media.
"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," he wrote on the social networking site.
The clash came just hours after Bieber said he was "getting better" following breathing problems he suffered during the previous night's concert. The star took a short break to go backstage, where he was given oxygen, and had to be briefly hospitalized for a check-up.
A spokesman for the O2 Arena said Bieber was treated backstage during Thursday's concert after becoming short of breath, but recovered and finished his set.
"He was treated by our team of medics and after further examination they didn't find anything more serious or worrying."
A spokeswoman for Bieber said he was feeling "a little under the weather." She demanded anonymity to discuss the star's condition.
Bieber later posted a shirtless photo of himself in a hospital bed, saying he was getting better and listening to Janis Joplin. Before that on Twitter he thanked "everyone pulling me thru tonight."
"Best fans in the world," he wrote. "Figuring out what happened. Thanks for the love."
Video footage from the concert shows Bieber appearing to fade during a performance of his up-tempo hit, "Beauty and a Beat." He slows down, puts a hand to his head then bends over, resting his hands on knees before walking slowly to the back of the stage.
The AP spoke to 18-year-old journalism student Prithvi Pandya, who shot the footage, to confirm its authenticity.
"When he started `Beauty and a Beat' you could see he was struggling," said Pandya, who was near the front of the crowd. "He took lots of drinks of water, that seemed unusual, and he was really sweaty, sweating a helluva lot.
"Toward the end of it, he went backstage. We didn't see him fainting. They brought on dancers to entertain, and I knew something was wrong at that point."
Bieber's manager, Scooter Braun, appeared onstage and told the crowd that the singer was feeling "very low of breath" but would come back to finish the show.
Jazz Chappell, a 20-year-old concertgoer who brought her younger sister and her friend to the show, said that In the nearly 30 minutes he was offstage, some fans started to leave. Once his manager announced what had happened, Chappell said many fans in the audience were gasping and crying, while others kept cheering for him to return.
"I thought, `Give the guy a break. He just fainted. He's not a performing horse. Let him rest a second,'" said Chappell.
Chappell said Bieber, who is in London to perform four concerts at the O2, later returned and performed low-energy renditions of his hits "Boyfriend" and "Baby."
The incident caps a difficult week for Bieber. He was forced to apologize to outraged fans who accused him of taking the stage almost two hours late for his first concert at the O2 on Monday. He insisted he was only 40 minutes late and blamed "technical issues." He took to Twitter to vent his frustrations with the media's portrayal of the incident.
The star's Believe world tour is due to move on to Portugal on Monday, then continue across Europe, the Middle East, South Africa and North America until August.
AP writers Gregory Katz in London and Derrik J. Lang in Los Angeles and AP Music Writer Mesfin Fekadu contributed to this report.
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