Abstract: An intruder detection and warning system has a plurality of infrared sensors for receiving infrared radiation from respective contiguous areas of premises in need of security. A central illumination unit including a rotatable light is operated to project a beam illuminating a selected one of the contiguous areas of the premises in response to the received infrared radiation by the infrared sensor in the selected area indicating presence of a person in the selected area. Initially a polite message randomly selected from a group of different polite messages is broadcast to request the intruder to leave the premises. When continued presence of the intruder is detected, the light beam is moved away from and then back to the selected area and a further message randomly selected from a group of different more demanding messages is broadcast to demand the intruder to leave the premises.