The invention relates to the field of building, and more particularly to the design of protective safety devices, and can be used for protecting openable apertures in windows installed in a variety of different types of buildings. The aim of the invention is to expand the existing range of design solutions, and more specifically to create a universal protective window grille while achieving a novel technical result that lies in the possibility of adjusting the geometrical dimensions of the grille prior to installation in different-sized openable window apertures, i.e. creating an extendable grille. The invention also solves the problem of increasing the functional properties of a grille, and more specifically of allowing the repeated use of old removable mosquito screens. The presence of a movable centre part makes it possible in certain situations, in the event of split pin fastening, to move said centre part by pulling out the split pins to free oppositely arranged guard elements, and to clear a passage by bending the bars to create an emergency exit, for example in the event of a fire.