A pack of pissed-off party girls turned Halloween into a full-on fright fest... exploding into a wild brawl that triggered a police response.

TMZ got the clip -- and it all popped off outside the restroom at the Unruly Agency/Memehouse Productions bash at CBS Studios Saturday night ... where a pirate-clad partygoer was caught shoving a blonde dressed as Buzz Lightyear, who spun around in shock and snapped, "Bro, what the f**?" TMZ.com

Buzz’s sidekick -- rocking a look inspired by Jessie from Toy story -- then came in swinging, smacking the pirate right in the face... only for the pirate to fire back with some serious, adrenaline-fueled shots of her own. TMZ.com

Honestly, it was pure chaos and dragged on way too long -- and turns out security was nearby the whole time ... finally

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