1. A method of detecting the occurrence of an event in a crowd based on analysis of video footage of the crowd, the video footage comprising a series of image frames, the method being performed by at least one processor and comprising the steps of: (a) receiving the video footage at the at least one processor; (b) calculating, for each of the series of image frames, a complexity value indicative of overall crowd movement dynamics, the complexity value being calculated in dependence on said each of the series of image frames, thereby calculating a series of complexity values; (c) analysing the series of complexity values to identify any changes occurring among the series of values; and (d) in response to identifying a change among the series of complexity values having a magnitude in excess of a threshold, transmitting an output signal to alert an operator that the event has been detected.