Jamie Lynn Spears — Knife Wielding Badass … Caught on Security Camera (VIDEO)

EXCLUSIVE

We got the video — Jamie Lynn Spears freaking out with a huge knife as she tries — and succeeds — in breaking up a sandwich shop melee.

TMZ broke the report … Britney Spears‘ lil sis was in the Pita Pit in Hammond, LA last month when a female friend got trapped in the middle of a bunch of guys brawling. Now we…