A flash mob hit Macy's in Center City Saturday - 650 vocalists who, unbeknownst to shoppers, had arranged to burst into song at noon.

Buyers in women's shoes were startled.

A customer in handbags smiled as a man in line next to her opened up with the "Hallelujah Chorus" from Handel's Messiah.

Men perusing suits tried hard to pretend nothing special was going on.

Others were so moved by the stunt they choked back tears.

"I was a little verklempt, even knowing it was coming," said Gary Steuer, chief cultural officer for the City of Philadelphia.

The "flash opera" happening was organized by the Opera Company of Philadelphia, which started putting out coy clues on Facebook and Twitter a few days ago.

Word was sent to area choruses and churches, and by Friday 28 groups had signed up. They assembled in rehearsals Saturday morning in the Wanamaker Building's Greek Hall, then were sent into Macy's before noon to be girded by Peter Conte and Fred Haas in a four-hand arrangement at the mighty Wanamaker Organ.

"It was phenomenal - really unique," said Gloria Elnitsky of Fishtown. "I'm thinking, 'Where are those voices coming from? They're coming from all over the place.' "

"It's a great way to get music out to people, and the more acts you do when you bring music to the public, the more it might spark an interest," said Adenike Webb, a Mount Airy member of the Philadelphia Singers chorale and one of the volunteer stealth singers.

The Opera Company is preparing a video of the seemingly spontaneous "Hallelujah Chorus" and plans to post it on YouTube by late Monday.

"This isn't about audience development per se. It isn't about harnessing new technology per se. They are collateral benefits," said the Opera Company's executive director, David B. Devan. "What is at the heart of this is bringing joy into people's lives with the widest platform possible."

(Devan artfully acknowledged that the iconic chorus from Handel's oratorio, composed in 1741, isn't opera: "Although opera is at the core of what we do, there are some soft edges around that.")

The Messiah at Macy's was the encore to an ambush the Opera Company staged at the Reading Terminal Market in April. Singers dressed like any other Saturday morning cantaloupe thumper fanned out across the selling floor and, on cue, broke into the "Brindisi" from La Traviata.

Shoppers froze, visibly tickled or moved by the stunt, or went about their business as if nothing out of the ordinary were happening.

No one tracked whether that event, which was inspired by a similar one in a market in Valencia, Spain, stimulated ticket sales to the Opera Company's concurrent La Traviata at the Academy of Music. But it was 31/2 minutes that reverberated beyond the food marketplace to a larger cultural one. A video, including reaction shots, was posted on YouTube, where it has clocked nearly three million views in six months.

The post elicited hundreds of e-mails.

"The thing that most impressed me was the geographic range," said Opera Company spokeswoman Tracy Galligher. "A teacher in Australia who was showing it to his class as an example of innovative use of the arts, a music lover in Brazil who wanted to do this in their own marketplace, lovely e-mails from Germany, Thailand, West Coast and Midwest U.S., Canada, U.K. The universal theme in the e-mails . . . is about the power of music and the common bond it creates. There were many 'this gives me hope' notes."

MrZoff01 wrote that "if everyday life could only be so beautiful . . . can you imagine? This is awesome . . . flat awesome!!!"

"Omg, that was so much fun. Now I know why I miss my hometown so much," wrote BartSRQ.

Donations to the company arrived specifically citing the "Brindisi" video.

It also caught the eye of the Knight Foundation, which recognized how nicely the event dovetailed with its own Random Acts of Culture program, which aims to sponsor 1,000 events like this nationally in the next three years.

Knight decided to support more such Opera Company of Philadelphia events with a $30,000 gift.

Dennis Scholl, vice president of arts and Miami program director for the Knight Foundation, said that by the end of the week, 60 similar Knight-funded events will have occurred here and in Detroit; Miami; St. Paul, Minn.; Charlotte, N.C.; Macon, Ga.; Akron, Ohio; and San Jose, Calif.

"This is an opportunity really to get people to think about opera and to think about classical music and think about ballet in a different way - the randomness of it, almost the subversiveness of it," said Scholl. "If you've seen any of the videos, you see the unmitigated joy on people's faces."

The Opera Company is plotting more of the same for Philadelphia through June.

Where? When?

As the narrator in Luciano Berio's Sinfonia puts it:

We must collect our thoughts, for the unexpected is always upon us, in our rooms, in the street, at the door, on a stage.

Or, as the Opera Company's Devan said rather less mysteriously:

"Expect us to pop up somewhere near you."