The fiber optic intrusion monitoring system comprises a multimode optical fiber connected to a coherent light source, a CCD detector 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 the action of 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. Each of the current images is subtracted pixel-by-pixel from the reference image. The output signal is obtained by summation of absolute values of all pixels differences. Intrusion monitoring system can find application in surveillance of perimeters, and industrial objects, deposits of chemicals and radioactive waste, etc.