This video is awesome! So this is an Off-Kai, or an offline meeting of online friends. I think these are people who "met" on a cosplay-for-fun board on 2-channel. They met up for real in the middle of an Akihabara street on a Saturday to perform this wonderfully synchronized dance with blasted stereo music. Look how happy they look!
Then the cops show up and the music stops and everyone runs.
To the lay person, this congregation might look like a harmless street dance unworthy of triggering the fear of cops, but you have to understand that it took a huge amount of guts for these law-abiding Japanese citizens to decide to do this without permission. I can’t imagine the cops in the US ever having this kind of power over the people, even with their fancy guns and booming voices. Sooo awesome.