Jamie Lynn Spears Knife Wielding Badass ... Caught on Security Cam

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 story ... Britney Spears' lil sis was in the Pita Pit in Hammond, LA last month when a female friend got caught in the middle of a bunch of guys brawling. Now we have surveillance video of JLS playing peacemaker ... while waving a 7-inch blade!

It's a wild scene -- she pulls her gf out of the scrum ... then, when the dudes keep going at it ... Jamie Lynn, mother of a 6-year-old, struts behind the counter, grabs the knife and waves it around. While she never cuts anyone ... the threat was enough to break up the fight.

Chillest dude in the whole mess though is the pita clerk who calmly blocked Jamie while calling for backup.

Only question -- why didn't she whip up a pita while she was at it? Big mistake, Jamie Lynn. Huge.