In the city of Sabadell in Spain's Catalonia region, a lone bassist in the Plaza Sant Roc is soon joined by a cellist and then gradually the rest of an orchestra and chorus to perform the Ode to Joy from Beethoven's Ninth Symphony, one of the best-loved musical compositions in the world. Beethoven's Ninth is traditionally performed by orchestras across Japan on New Year's Day (for example here), and the Ode to Joy itself is the anthem of the European Union.
The event was sponsored by a local bank, Banc de Sabadell, in celebration of its 130th anniversary in 2011. The performers are from the Vallès Symphony Orchestra and the Lieder Chamber Chorus, Friends of Opera Chorus, and the Belles Arts chorale.
For another, far chillier, but still very entertaining musical flash mob, see this joyful wedding celebration in the Sparrow Hills section of Moscow this past February.
(Updated 2017 July 25 mainly to add additional links.)