Justin Bieber -- Homecoming Party Busted Up by Cops

Justin Bieber just got back to the U.S.A. and he's already got police pounding on his front door -- for throwing one of his trademark Beverly Hills all-nighters.

According to law enforcement sources ... Bev Hills P.D. went to Bieber's condo TWICE last night to deal with noise complaints. The first visit was shortly after 11 P.M. ... but when officers arrived they didn't hear anything, so they left without knocking.

The second time -- around 12:40 A.M. -- there was noise ... we're told around 30-40 people were milling around Bieber's balcony. Cops knocked on the door, and spoke directly to Justin ... reminding him his neighbors could make a citizen's arrest if the noise continued, but Justin was cooperative and agreed to break up the party.

As we first told you ... JB was supposed to be out of this building at the end of September, but he's been traveling around Europe for several weeks ... and hasn't found a new pad yet.

Enjoy him while you can, Beverly Hills.