A ROOFTOP sniper taking aim on 31st St. caused a stir yesterday, but turned out to be an actor filming “Law & Order: Criminal Intent.”

A lunchtime crowd was startled around noon when the actor was seen poking the barrel of a long-range rifle over the roof of a building between 5th Ave. and Madison.

“It had absolutely nothing to do with the sniper in Maryland,” a “Law & Order” spokeswoman said.

The scene was finished by late afternoon, she said.