The singer was nearing the end of a private show in front of a packed house when the power for most of the stage instruments suddenly went out, said his manager, Scooter Braun.

“Then we hear the fire alarm,” Braun said by phone Monday evening. “Literally, the boy blew up the Apollo – he heated up the Apollo.”

But Bieber still managed to finish his show, with the help of fans singing along with him.

“What it turned into was an Apollo moment, like one of those impromptu things,” said Nina Flowers, the Apollo’s rep.

“It was one of the most special nights I have ever been a part of,” said Braun.

The power outage, which only affected the stage instruments and some lighting, occurred toward the end of the show. Braun said they were told the problem was supposed to be only a few minutes, but it stretched to more than 40 minutes.

At first, Bieber took the outage in stride.

“He was joking, he said it was all the hot girls in here,” said Flowers.

He then got on the drums, which were working, for a solo, and had the crowd serenade Braun for his birthday.

But then Bieber got upset at the disruption of the show, which was being taped as part of an NBC special to air later this week.

“He started to get frustrated, because it had been 40 minutes,” said Braun.

However, instead of getting mad, Bieber decided to put his energy into performing for his fans, who stayed in the audience during the delay.

“He walks out, he literally quiets the crowd, and he says, ‘I’m sorry the power is out. . (But) you guys have always had my back. I’m going to sing ‘Boyfriend’ and you’re going to sing it back to me.’“

He then launched into his hit song from his new album Believe as the crowd of mainly teen girls sang along as he danced, Braun said.

Flowers couldn’t say what caused the power outage, but she praised the 18-year-old entertainer for finishing the show.

“He could have stopped; he could have not gone on and said, I did everything . but he came back on,” said Flowers.

She added that Bieber showed reverence throughout the night for the famed theater, a hallowed hall where the likes of James Brown, Stevie Wonder and Bieber’s idol, Michael Jackson, have performed.

“It was really like a special moment,” she added.

Bieber’s concert was part of a series of promotional appearances to push his album “Believe,” out Tuesday. It has garnered strong reviews. Braun said he’s hopeful the album will prove that Bieber has more to him than just hype.

“Every step of the way he’s been doubted,” Braun said. “‘‘He is a great artist and entertainer and tonight he showed that.”