Justin Bieber's week just got worse: following a brief hospital stay after fainting backstage, the 19-year-old pop star's preparation for a final concert in London on Friday hit a speed bump when he got into an altercation with insult-hurling paparazzi.

Bieber responded to the insults by lashing out at a photographer with a stream of expletives as he was restrained by minders.

The singer quickly took to Twitter to address the incident, which was captured by Channel 5 news, and pledged to channel his "adrenaline" into his Friday show.

"Ahhhhh! Rough morning. Trying to feel better for this show tonight but let the paps get the best of me," he posted on the social networking site, where he has more than 35 million followers.

"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."

Friday show to go on

The scuffle came just hours after Bieber said he was "getting better" after struggling to breathe during the previous night's concert at the O2 arena, and pledged the show would go on as planned. A spokesman for the O2 Arena said Bieber had been treated backstage during Thursday's concert after becoming short of breath, but recovered and finished his set.

"As far as we are concerned everything is on, on, on" for Friday's show, Jeremy King said.

"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 recovering after Thursday's incident, which saw him given oxygen before returning to the stage.

"Justin has been released from the hospital after a check-up and, while he's feeling a little under the weather, he's currently planning on going ahead with tonight's show," the spokeswoman said Friday. 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."

Faded mid-concert

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.

Braun later tweeted "everyone please give him a little space and he will be ok. Im sure he appreciates the support ... Tough kid proud night once again he always finishes the show. Full out. True pro..."

RELATED: Bieber apologizes for late London appearance

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.