Here's an idea of just how insane it was:

At least 500 fans gathered outside, chanting Justin's name, forcing police to close the surrounding streets. Police also told Justin to remain in the hotel and stay away from the balconies.

According to Justin's rep, the cops said that if he didn't stay inside, he and his people could be arrested for INCITING A RIOT.

On Twitter, Justin said, quote, "There are like thousands of people out there. Love everybody, but gonna try and get some sleep. Please don't scream.  LOL."

He later added, quote, "THIS IS CRAZY! I'm trying to work it out so I can say 'Whatup' to my fans. I LOVE YOU GUYS!!"

Justin was eventually permitted to wave to the fans from his balcony. He Tweeted, quote, "That was great!  Got to say hi to everyone.  We are following the rules.  LOL."

The police seemed to be under the assumption that if they allowed Justin to wave, everyone would go home. Of course, that didn't happen…it just kicked the hysteria up a notch. At least 25 officers were on the scene trying to control the crowd.

A police spokeswoman said they "recommended" that Justin stay in his room until things calmed down…but denied that they threatened Justin's camp with arrest…adding, quote, "I wouldn't say it's a riot situation, yet. Officers are working with the management of the hotel and with Justin Bieber's security team to ensure his safety, the safety of other guests and of the crowd."

Justin is in Liverpool for a concert. He's performing there tonight.

Here's video of this INSANITY. Here's a video of the crowd chanting "We want Justin!" and here are a few pictures, including the one of Justin waving to the fans from the balcony.