Rams star Bobby Wagner gave security guards a hell of an assist during "Monday Night Football" ... knocking the snot out of a fan who had invaded the field with a smoke bomb in the middle of the game.
The wild scene happened just before halftime of L.A.'s loss to the Niners in Northern California ... when a protester hopped over Levi's Stadium railing and began running wild on the turf.
The man -- holding a smoke bomb that filled the air with pink fumes -- managed to evade stadium staffers for several moments .