'STAR WARS' DAY STORMTROOPER TAKEN DOWN BY COPS Over Plastic Toy Blaster

The Lethbridge Police Service tells TMZ ... a service investigation into the actions of the officers who responded to the firearm call Monday has been launched.

We're told the female in the Stormtrooper costume dropped her "weapon" as soon as cops arrived, but "did not initially comply with further police directions to get down on the ground."

Cops say she sustained a minor injury during the incident but didn't need medical attention. The firearm was confirmed to be fake and she wasn't charged.

Nonetheless, Chief Scott Woods has initiated an investigation after reviewing additional information -- including social media video -- to determine whether the officers acted appropriately. -- Watch live at https://www.twitch.tv/werenotsorry