Justin Bieber's fans are "crazy", says actor Falk Hentschel.

Falk experienced the singer's followers when they starred opposite each other in the 2011 CSI episode Targets of Obsession in the eleventh season of the hit TV series.

The actor was blown away by Justin's fame.

"Yes I've experienced the Biebs phenomenon," Falk told Cover Media.

"It was a crazy, crazy experience with his fans. We had paparazzi sitting in the corners around us."

The German actor had no idea who Justin was at the time. He added the singer was a "sweet kid" and enjoyed working with the star.

"I sort of knew of him but I didn't know he was Justin Bieber. All I knew was he was this kid who sings which probably helped," Falk said laughing.

Falk is now preparing for the release of his new film Street Dance 2 which is release in UK cinemas on March 30. He plays the role of Ash, a dancer who travels through Europe gathering talent to form a group for a competition in Paris.

Meanwhile, Justin's followers have been in a frenzy over the release of his new album Boyfriend in a week.

"we got #1WEEK #7DAYS until we try and make history. workout, rehearsal, dinner, studio. IM FOCUSED (sic)," Justin wrote on his Twitter account Monday.

The announcement was followed by excited tweets from punters with one fan writing: "@justinbieber I'm really excited this album #BOYFRIEND1WEEK:) (sic)."