Unruly personnel, e.g. rioters, are subjected to continuous low intensity illumination of one part of the visible spectrum, which is sufficient for identification purposes, simultaneously with flashes of high intensity light of a different part of the visible spectrum, the latter is sufficient to blind the eye temporarily to the low intensity illumination and cause movements to appear confused and chaotic in the flashing high intensity illumination, whereby the activity of the crowd is inhibited or arrested. Police equipped with goggles, which are transparent to the continuous low intensity illumination but opaque to the flashing high intensity illumination, can see adequately under such conditions to remove individual agitators.