(TMZ) - 0306_bieber_getty_articleJustin Bieber was rushed to a hospital in London moments ago ... after collapsing backstage during a concert... TMZ has learned.

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

Bieber 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 on 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."

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 he's still there receiving treatment. 

TMZ reports he's in good spirits and feels OK... but doctors want to evaluate him. 

Bieber didn't appear too sick last night... when he was running around London while sporting a gas mask.  Maybe he should leave it at home from now on.