(Jim Dyson/Getty Images)

Justin Bieber isn't even out of his teens yet, so he should understand that starting a concert late on a Monday night just isn't cool. British papers slammed the singer for taking the stage at London's O2 arena at 10:35 p.m. - two hours past the advertised time - on a school night.

Fans began booing as Bieber's two opening acts came and went, and they waited - and waited - for the singer to appear, The Mirror reported. Angry parents began tweeting their displeasure, saying things like, "Kids are having to leave as it's so late. Not fair," and, "How rude. Bieber is still not on stage at the O2 when his concert was due to finish now! Mate, it's a school night."

Another tweeted, "Who the hell does Justin Bieber think he is? Not on stage yet?! I hope everyone walks out of the O2." And yet another wrote, "Think of the children #biebergeddon. Bieber concert at the O2 due to finish in 5 minutes, he's not even on stage yet."

There were also concerns that the tube - the British subway system - would stop running before the end of the show, stranding fans at the arena.

Finally, at 10:36 p.m., the O2 arena itself apologized for the late start, tweeting, "Justin Bieber is now on stage and apologizes for the lateness of his show. It added, "Sorry to all the Justin Bieber fans for the lateness of his show tonight. The tube will still be running when the show finishes."

One angry parent responded to that tweet, "Disgusting my daughter has had to leave after 1 song to ensure she gets home a total and utter disgrace. Your apology is pitiful !!!"

One witness recounted to E! News, "I was at the concert and he [Bieber] came out at 10:15 p.m and Carly Rae Jepsen finished her set at 9:10 p.m….Justin should have been out at 9:30." The frustrated fan added, "Everyone was kept waiting not knowing anything. All they did was kept playing Michael Jackson songs. All the fans were getting so annoyed at around 9:50 p.m., fans started booing."

The female fan continued, "He didn't even apologize. That's what makes it worse…My mom paid for five tickets so she's not happy."

Yet another concertgoer tells Us Weekly that Bieber cut short his set list due to his late arrival on stage.

"last night i was scheduled after 3 opening acts to go on stage at 935 not 830 but because of some technical issues i got on at 10:10..so…" Bieber tweeted after the show.

"i was 40 min late to stage. there is no excuse for that and I apologize for anyone we upset. However it was great show and Im proud of that," he added.

The Biebs is set to perform again this evening.

"and tonight we will run on time and look forward to putting on an amazing show for everyone in attendance," he tweeted.

"i never have any intent to upset or let anyone down. and Im not okay with things being exaggerated. once again sorry for anyone upset."

His final tweet about the show said, "now we have a show to do today and Im gonna focus on the positive and put on a great show tonight here in London."