Justin Bieber was BOOED when he took the stage last night for the first of four shows at London's O2 arena . . . because he was TWO HOURS LATE.

The show was supposed to start at 8:30, but he didn't get out there until 10:30.  A lot of younger fans had to leave before the show even started.  There's no word why it was delayed.

 (--Justin's rep is denying this.  He says Justin was only about half an hour late, and nobody booed.)