Mill hosted the massive Grammy after-party Sunday night -- Nicki Minaj, French Montana, Khloe Kardashian, Chris Brown and Justin were at the bash ... and sources inside tell us it got crazy when a fight broke out between rival gang members.
During the melee, we're told punches and even ice sculptures were tossed -- and most of the guests fled into the street. Law enforcement sources tell us ... cops were called around 1:30 AM for loud music, and found more than 1,000 people walking around the ritzy 'hood. It took about 3 hours to clear the streets.
No arrests were made, and police never went inside the house -- which was totally trashed. We're told Justin moved out last week, and Meek moved in ... signing a 1-week lease to lock down the party location.
We're guessing he can kiss that security deposit goodbye.