The pranksters at Improv Everywhere describe their latest noble work thusly:
For our latest mission, 16 agents staged a spontaneous musical in the food court of a Los Angeles shopping mall. We used wireless microphones to amplify the vocal performances and mix them together with the music through the mall's PA system. We filmed the mission with hidden cameras, mostly behind two-way mirrors. Apart from our performers, no one in the food court was aware of what was happening.
Link to their blog post with more details and pix. (thanks, White Male in Business Attire!)