You know how much I like this stuff. From the YouTube description:
"On Saturday, October 30, 2010, the Opera Company of Philadelphia brought together over 650 choristers from 28 participating organizations to perform one of the Knight Foundation's "Random Acts of Culture" at Macy's in Center City Philadelphia. Accompanied by the Wanamaker Organ - the world's largest pipe organ - the OCP Chorus and throngs of singers from the community infiltrated the store as shoppers, and burst into a pop-up rendition of the Hallelujah Chorus from Handel's "Messiah" at 12 noon, to the delight of surprised shoppers. To learn more about this program and view more events, visit the Random Actos of Culture website."
My thanks to Jan d'O for bringing it to my attention.