Justin Bieber's Fans -- Hotel Under Siege!

While there's no way of ever knowing if Anne Frank would have been a Belieber, it's pretty safe to say guests staying at the same hotel in Norway as Justin Bieber are not fans.

Hordes of fanatical Beliebers staked out the Oslo hotel JB was staying at in hopes of catching a quick glimpse of the teen pop wonder -- and his new ladylike 'do -- before his concert on Wednesday.

Although not mentioned in the brochure, all rooms come with a view ... of hundreds of screaming Nordic tweens.