Justin Bieber excitedly tweeted about his 19th birthday bash, referring to a "big night ahead," -- which, by the end of the night, turned into his "worst birthday." So, what went wrong?

An eyewitness told E! News that the singer held a birthday party at London's Cirque du Soir Friday with his rumored new galpal, British singer Ella-Paige Roberts Clarke.

Things turned sour, though, when members of Bieber's entourage were confronted by club security, sending his entire party packing. The incident came to a quick end however, and Bieber returned to his hotel afterward, the source says.

Disappointed fans flocked to Twitter using the hashtag "#BeliebersHatePaparazzi," blaming them for ruining Bieber's party.

Representatives for Bieber had no comment.