Bieber pulled himself together to finish the concert before going to a hospital to be checked out, TMZ said.
A source the entertainment website described as close to the Canadian teenager said Bieber felt woozy during his performance at the O2 Arena and stumbled backstage where he collapsed with four songs to go in his set.
TMZ said Bieber was brought into a private room where emergency personnel treated him with oxygen. Sources said the singer was advised to go to a hospital, but he insisted he "had to finish the show for his fans."
Bieber's representative, Scooter Braun, went on stage and told the audience Bieber was suffering from "very light breath."
"The whole show he's been complaining," Braun said. "He's backstage with the EMTs and the doctor."
Bieber returned to the stage to finish the show, drawing a resounding ovation, TMZ said.
He then was taken to a hospital where TMZ said he was feeling OK while being evaluated by doctors.