This is an Operatic version of "Flash Mob" (the latest craze), that I would like to call Ambush Opera or Guerilla Opera.
Suddenly in the least expected place, from amongst the shoppers, performers without warning break into song/opera.
It is Entertaining, for the performance, and to see the astonishment, and pleasure of the crowd.

On Saturday, April 24th, over 30 members of the Opera Company of Philadelphia Chorus and principal cast members from the upcoming production of La Traviata converged on the Reading Terminal Market Italian Festival.  Wearing street clothes and blending in with the crowd, the artists swung into action after the first orchestral strains of the famed "Brindisi" were piped through the market, giving a rousing, surprise performance for hundreds of delighted onlookers who were there to enjoy the Italian delicacies and the everyday treats that the Reading Terminal Market has to offer.  The four-minute piece won a thunderous ovation from the audience.

The name of the piece is "Libiamo ne' lieti calici" sometimes referred to as the "Brindisi." It means "Drinking song." 
The opera is "La Traviata." Music by Giuseppe Verdi. Libretto by Francesco Maria Piave. The chorus takes place in Act 1 of the opera. 
There are Dozens of wonderful "Staged" Versions of "Brindisi"on You Tube
But I like the one below that has a more Intimate Setting , and has Accompanying Translation
Placido Domingo and Teresa Stratas sing the Brindisi from Act I of La Traviata directed by Franco Zeffirelli.
 

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