"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."
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.
AP writers Gregory Katz in London and Derrik J. Lang in Los Angeles and AP Music Writer Mesfin Fekadu contributed to this report.