A fiber optic intrusion monitoring system comprises a multimode optical fiber, a coherent light source, a CCD, and a processor for generation of the output signal. The system is based on the principle of variation of speckle pattern in the far-field of a multimode optical fiber under mechanical perturbation. By processing the speckle pattern, one can derive information on the amplitude of perturbation. The algorithm for processing of the speckle pattern is based on comparison of the current speckle image with the reference image. An intrusion monitoring system can be applied in surveillance of perimeters, industrial objects, deposits of chemicals and radioactive waste, etc.