Justin's concert in London Monday was slated to start at 8:30 PM ... but he didn't take the stage until 10:30 PM -- and a very young and pissed off crowd roundly booed his skinny ass.
Particularly irritating ... most of his fans have a curfew, which put them in a bad position.
Bieber tried to defend his lateness by tweeting he was actually only 40 minutes late because the opening acts were always scheduled to last more than an hour.
Bieber admitted there was no excuse for being late, but then blamed the media for spreading the word of his tardiness.
Better watch out Biebs -- fame is a fleeting thing.