I visited DPRK in September 2013 and was lucky enough to see the Mass Games twice. The show has around 100,000 performers and it tells the story of The Korean Revolution through synchronised dance, gymnastics and acrobatics. The giant picture on the far side of the stadium is created by 20,000 children, all with books of coloured cards, each book containing up to 170 pages. The scene at the start is their warm up. The show runs each year from July to September or October; you can watch as many videos online as you like, but to truly appreciate it, it's best to see it live.
Edit...you can now download this.