A flash mob is a very fast mobilization, a gathering of a group of persons, in a public place to create something in particular before scattering/dispersing quickly.
Improv everywhere , a new yorker collective made this term fashionnable again by organizing "freezings", meetings aiming an immobilization in a public place for an instant, to create the impression that time has stopped, frozen.
These freezes happens in New York, London, Brussels, and this wednesday march 19th, in Paris at gare de Lyon.
It was designed to start at 10.42 pm and the freeze supposed to last for 90seconds. Between 200 and 300 persons were expected... watch the result !