From typicallyspanish.com

Spanish Oddities
Commuters come to a complete standstill in Madrid on purpose
By h.b.
Apr 8, 2008 - 2:19 PM

A group of some 100 people gathered at 8,30pm at the Atocha railway station in Madrid on Monday night, just to stand still.

The gathering had been planned and publicised over the internet and at 8,30pm on the dot a whistle was sounded indicating people should then stand still for a period of five minutes.

People took up different and interesting postures, some even in costume, and smiles were wide as the experiment took place.
What nobody could quite explain was, why?