Chris Brown Ditches Migos Pre-GRAMMY Party When Cops Show

Chris Brown joined the Migos for a pre-GRAMMY turn-up that got way too lit ... and then cut out like he was America's most wanted when cops showed up.

The Migos were hosting a private "Black Tie GRAMMYs" mansion party Saturday night in Bev Hills -- with the R&B singer in tow and his main lady, Vanessa Vargas, at his side.

We're told around 1,500 people filtered in throughout the night, since the address was listed publicly. Eyewitnesses tell us it was PACKED -- so much so that Chris had to take a seat on some blond chick's lap for a while ... as Vanessa watched from a distance.

Sure enough, cops were called around 1:15 AM and spent almost 2 hours breaking up the party and writing up tickets. We're told soon after they showed ... Chris was already riding away in his whip. Hard to tell if Vanessa was with him.

Seems like he wasn't trying to deal with more cops, especially so early in 2017. Can't blame him entirely though ... the place looked chaotic as hell.