This is a West Side Story flash mob in Times Square, shot on Friday by our man Andy Dallos. I'm spoiling it for you because it takes a little while to warm up, so if you lose interest in the first 30 seconds you miss it.
It starts with what appears to be a relatively modest street performance, but then somewhere around 1:00 it grows a little (watch again and look for the joiners), and then somewhere around 1:40 or so, I can't tell exactly where, enough people are involved that it really does qualify as a "mob."
The only thing it needs at the end is a ninja smoke bomb, but disappearing back into the crowd is pretty effective, too.