This paper presents a method to detect passengers on-board public transport vehicles with the ultimate aim of monitoring their behaviours under suspicious circumstances. The method comprises first an elliptical head detection algorithm using the curvature profile of the human head as a cue. This is followed by applying the geometric blur features which are consistent to affine distortion of the image to keep track of the movement of the head within the vehicle. The profile of the moving heads with respect to each other within a length of time can then be used as indicative features to detect the advent of suspicious behaviour of the passengers.