You don't want to mess with Jamie Lynn Spears.

That's what patrons at a sandwich shop in Hammond, La. found out last weekend when the actress and country singer stepped in to break up a fight, and reportedly ended up waving a knife around.

According to TMZ, the altercation happened at the Pita Pit in Hammond, where Spears and a friend made a late-night food run. The celebrity gossip site reports that another patron attacked Spears' friend, hitting her with a bottle.

Sources close to the singer say that Spears leaped into action, pulling her friend behind the counter to safety -- then snatched up a long, serrated bread knife and started waving it around to discourage any further problems.

Police were reportedly called to the scene, but Spears' friend declined to press charges, and the incident resulted in no arrests.

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