At exactly 3:30pm on a secret cue, almost everyone in the square froze. The few bewildered tourists didn't know what was happening. For 5 minutes the participants held their positions, and then magically everyone unfroze.

Why? Well, why not.

I think this one is better than the grand central station freeze prank. There's more people and the reactions of the people are funnier.

I would love to see an entire college campus do this, and watch the teachers and administrators go crazy trying to figure out whats going on. I think a reasonably medium sized college could pull it off, the more city-sized ones, not so much.