While I agree with the need for an armed citizenry as the first line of defense against those who would deprive us of liberty, a sniper presents a unique challenge under any circumstances. Perhaps someone more versed in tactics than me can suggest what type of response a few folks armed with handguns can mount following a sniper attack. I can't think of anything except to be prepared to stop and detain someone looking suspicious in the general area from which one or more shots was heard. (For example, perhaps, someone climbing down a fire escape of a tall building with a sniper rifle strapped across his back?)
How about this: An armed citizen walks around the corner and sees a man holding a rifle a 30 yards away. The man fires, and down range it can be seen he hit an unarmed person. Armed citizen pulls out handgun and shoots terrorist 5 times.
The only way this will stop is for a armed good guy to be there when it happens, see it happen, and be close enough to return fire. The odds of this are much greater the more armed good guys are out there. The police are not enough.