I don't recall where I first heard about this, but it's now traditional in Japan to perform Beethoven's Ninth Symphony at New Year. According to Mental Floss (link), the tradition dates back to World War I, and it's become so popular that in Japan the symphony is referred to simply as daiku, "Number Nine."
In Osaka, a group of mostly amateur singers called the Number Nine Chorus get together every year to give a performance involving 10,000 voices. (That's not a typo. With that many people they might have to take the speed of sound into account to keep them in sync.)
On 2011 March 11 a tsunami devastated parts of the city of Sendai, about 600 kilometers northeast of Osaka. At the end of the year a chorus in Sendai was invited to join the Number Nine Chorus in performing daiku by means of a remote television link, and in the video below you see both groups singing together. I believe the conductor is Yutaka Sado, a protégé of Leonard Bernstein and Seiji Ozawa, based on this blog post from Scott Brown. The same post identifies the soloists as Toshiko Abo, soprano; Masako Tejima, mezzo; Makoto Tananka, tenor; and Hiroshi Kuroda, baritone. I don't know whether the orchestra is an established one or one formed expressly for the performance. If someone knows, please pass it on in the comments and I'll update this post.
It's amazing to see. If you don't have time to watch the whole thing, at the very least skip ahead to about 6 minutes 30 seconds, and after a minute or two skip ahead again to 15:35. (Note the emotion visible on the face of the conductor when the performance ends.)
You might also want to take a look at an earlier post (link) involving a flash mob performance of an abbreviated Ode to Joy in the city Sabadell in Spain's Catalonia region.
(Update 2017 August 9 to note the conductor's name.)
(Update 2018 March 2: The audio on this clip has been missing for some time, possibly the result of a copyright complaint. The video is still a remarkable sight.)