Martial law: The coax gunner on that tank in the intersection can use you as target practice if you are outside at the wrong time.
Not exactly. It's Martial
Law afterall. This only means the military gets to do law enforcement duties using their equipment and men. That said, other measures are often taken along with martial law (such as curfews, suspension of civil rights, that sort of thing) that can result in a situation you described, and in fact did, in a particularly nasty way, just before the fall of communism in Poland.
That said, martial law can also be used without those measures, for instance if you need to take out a guy who stole a tank from a military base. Police isn't equipped with AT weapons, and the military can't be sent in under normal circumstances (this didn't actually happen, because the tank got stuck, but it was close).