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**?”

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.

Honestly, it was pure chaos and dragged on way too long — and turns out security was nearby the whole time … finally jumping in to bring an end to the fist fight.

Law enforcement sources tell TMZ … police got a call from security guards, who were standing with a female victim claiming she was assaulted by three women.

Cops rolled up to the party, but by then everyone involved had already cleared out … and the victim never pressed charges.