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5:50 AM PT -- We're told Justin has been released from the hospital. He feels crappy, but still plans on going ahead with tonight's show.

Justin Bieber was taken to a hospital in London moments ago ... after collapsing backstage during a concert ... TMZ has learned.

A source close to Justin tells us the singer felt woozy during the show at the O2 Arena ... and with 4 songs to go, he stumbled backstage where he collapsed.

We're told the singer was pulled into a private room where he was treated with oxygen from emergency personnel.

Sources say Bieber was advised to go to the hospital ... but declined ... insisting that he "had to finish the show for his fans."

That's when Bieber's rep, Scooter Braun, went on stage and told the crowd ... "Justin's got very light breath. The whole show he’s been complaining, He’s backstage with the EMTs and the doctor."

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A short time later, JB returned to the stage ... completed the show ... and got a HUGE ovation.

Justin was transported to the hospital directly following the show ... and we're told he's still there receiving treatment.