The invention relates to optoelectronics, namely fiber-optic sensors and protection systems and can be used to protect buildings and territories, depositories of chemical substances and radioactive waste, as well as military, industrial and other objects.The fiber-optic intrusion alarm system includes a coherent light source (1), coupled with a segment of multimode optic fiber (2), a photodetector (3) and a warning alarm shaper (5), in the capacity of which is used a computer, containing a numerical matrix differentiator (9) of two consecutive speckle patterns, an adder (10) of the corresponding difference signals of each two consecutive speckle patterns and a comparator (11), connected in parallel to a warning alarm trip level setting unit (12). The comparator (11) forms the alarm signal when the sum of difference signals of two consecutive speckle patterns exceeds the predetermined warning alarm trip level. The photodetector (3) is placed in the far field of the optical fiber (2) coaxially with it and is made as a CCD (Charge-Coupled Device) for recording of speckle patterns in the far field of the fiber (2) at a rate of F frames per second. To ensure the maximum value of the signal to noise ratio, the F speckle pattern frame recording rate is chosen according to condition: 1 ≤ F ≤ 5 frames per second.The result of the invention is to increase the operational efficiency, to reduce the probability of false signals and the effects of electromagnetic noises.