Poor Justin Bieber. The 'Right Here' singer spent his birthday (March 1) in London, but unfortunately, it was a rough night -- he even dubbed it "worst birthday." What happened -- and what did Jaden Smith have to do with it?

Bieber held a circus-themed birthday party in London's swanky nightclub Cirque du Soir with a bunch of pals, including rumored new flame Ella-Paige Roberts Clarke, and apparently it didn't end well at all.

E! Online reports that Bieber's entourage had an issue with the club security, leading to a confrontation between the guards and Biebs' party. It turns out that there hadn't been any mention of Jaden Smith going to the soiree. Smith is only 14 years old, and security for the club wouldn't let him in. (Maybe he should have used the "Do you know who my dad is?" line!)

Us Weekly reports that Bieber didn't want to betray poor Smith, so he had all of his pals leave the club in search of somewhere to grab grub -- and Radar Online says they wound up opting for McDonald's. (C'mon, Biebs! You're rich!)

An eyewitness said that it all went down very quickly, and that Bieber made a beeline for his hotel immediately afterward. And he wasn't happy about it, evidenced by his Twitter feed, which started out excited and ended dejected

(And with a ticket for Lil Twist driving his car to boot!) Keep your chin up, Biebs! We're sure your big 2-1 will be better than your 1-9. And his early celebration looked like a (shirtless) blast too!

Watch the Justin Bieber 'Right Here' Lyric Video Feat. Drake