Video surveillance systems produce huge amounts of data for storage and display. Long-term human monitoring of the acquired video is impractical and ineffective. Automatic abnormal motion detection system which can effectively attract operator attention and trigger recording is therefore the key to successful video surveillance in dynamic scenes, such as airport terminals. This paper presents a novel solution for realtime abnormal motion detection. The proposed method is well-suited for modern video-surveillance architectures, where limited computing power is available near the camera for compression and communication. The algorithm uses the macroblock motion vectors that are generated in any case as part of the video compression process. Motion features are derived from the motion vectors. The statistical distribution of these features during normal activity is estimated by training. At the operational stage, improbable-motion feature values indicate abnormal motion. Experimental results demonstrate reliable real-time operation.