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Justin Bieber says he's doing better after an onstage scare during his concert at London's O2 Arena Thursday night. 

The 19-year-old singer collapsed after experiencing shortness of breath but insisted on finishing his show - against doctor's orders - before he was taken to a local hospital.  He tweeted later that he was "getting better" and thanked fans for pulling him through. 

He didn't offer any explanations as to why he fell ill, only saying that they were still "figuring out what happened."  Eyewitnesses gave their account of what they saw from the audience to E! News.  During "Beauty and the Beat," the singer seemed to be walking toward the exit when he collapsed on the staircase.  His manager Scooter Braun then appeared with Bieber's friend, Jaden Smith, to inform the audience that the pop star was getting emergency medical attention and would return.  Braun told the crowd that the star had been complaining about being "very low of breath" throughout the show, and later tweeted that he was proud of his "tough kid," calling him a "true pro." 

Justin has had a rough week in London.  Last Friday he tweeted that he was having the "worst birthday" after a reported scuffle between members of his entourage and security at a London nightclub. 

On Monday he was criticized for being late for the first of his four scheduled shows at The O2, leading to rumors about his late-night partying and "diva" behavior, which he's denied. 

Justin was photographed wearing a black gas mask while exiting a popular restaurant on Wednesday night.  It's actually the second time the singer was seen wearing the mask, which some speculate may be his attempt to hide from the paparazzi.