A security system for constraining the degree to which a door leaf (11) may be opened; the system being selectively activated from an inoperative position in which it does not block the opening and closing of the door leaf (11), into and out of an operative position in which it does so block the opening of the door leaf (11) beyond a predetermined constrained degree; the system being characterized in that it comprises: a chassis (14) adapted for mounting, in use, adjacent an opening edge of a door leaf (11); an electric actuator (17, 28, 36) mounted on the chassis (14); an arm (18, 29, 37) operating in conjunction with said actuator (17, 28, 36) so that the actuator drives the motion of said arm (18, 29, 37) without any lost motion mechanism.