Just came across this: Opera Company of Philadelphia Rallies Over 650 Singers To Surprise Macy's Shoppers

Excerpts from the accompanying article (BF added by me):

On Saturday, October 30, 2010, the Opera Company of Philadelphia kicked off National Opera Week by partnering with Macy's and John S. and James L. Knight Foundation to produce one of Knight Foundation's "Random Acts of Culture" on a grand scale in Philadelphia.

With the Opera Company of Philadelphia Chorus as a core, over 600 singers from area choirs, accompanied by the famed Wanamaker Organ – the world's largest pipe organ – surprised shoppers at the Center City Philadelphia Macy's with a spontaneous rendition of the Hallelujah chorus from Handel's Messiah.   

The Opera Company is committed to encouraging the personal practice of singing in fun and accessible ways, and is honored to be performing with the revered Wanamaker Organ.

What I like best about watching the video is seeing so many folks joining in -- many with great gusto -- who (I think) are not actually choristers...