Closing system for through openings; comprising • a quick opening vertically moveable flexible panel, connected to an actuating motor driven drum • a pair of vertical guides secured to the wall or surface in which a through opening is defined, said flexible panel being assembled on said pair of vertical guides; • a bag containing a heavy element fixed to a lower end of the flexible panel, said bag being thicker than the flexible panel; • a fire door movable between an opening position in which the fire door keeps open the through opening and a closing position in which the fire door closes the through opening defined in said wall or surface; characterized in that • the vertical guides comprising a fixed lower part, arranged on each of the sides of a through opening and reaching the height of the through opening and, a mobile upper part, located above the through opening and vertically moveable between a lower position in which it is close to the fixed lower part and an upper position in which it is at a distance from the fixed lower part, defining an opening; • the mobile upper parts include abutments, the bag engaging said abutments and hauling the upper parts of the vertical guides towards said upper position as a result of an over-opening of the flexible panel by which said lower end of the flexible panel is raised above the through opening; • an element (61, 64) is movable by the fire door or by the flexible panel to a closed position in which said element (61, 64) is placed above the fixed lower parts of the vertical guides without interfering with the mobile upper part parts of the vertical guides when the mobile upper parts are in said upper position, closing of the space comprised between the wall and the fire door when this is arranged in its closing position. • a quick opening vertically moveable flexible panel, connected to an actuating motor driven drum • a pair of vertical guides secured to the wall or surface in which a through opening is defined, said flexible panel being assembled on said pair of vertical guides; • a bag containing a heavy element fixed to a lower end of the flexible panel, said bag being thicker than the flexible panel; • a fire door movable between an opening position in which the fire door keeps open the through opening and a closing position in which the fire door closes the through opening defined in said wall or surface; • the vertical guides comprising a fixed lower part, arranged on each of the sides of a through opening and reaching the height of the through opening and, a mobile upper part, located above the through opening and vertically moveable between a lower position in which it is close to the fixed lower part and an upper position in which it is at a distance from the fixed lower part, defining an opening; • the mobile upper parts include abutments, the bag engaging said abutments and hauling the upper parts of the vertical guides towards said upper position as a result of an over-opening of the flexible panel by which said lower end of the flexible panel is raised above the through opening; • an element (61, 64) is movable by the fire door or by the flexible panel to a closed position in which said element (61, 64) is placed above the fixed lower parts of the vertical guides without interfering with the mobile upper part parts of the vertical guides when the mobile upper parts are in said upper position, closing of the space comprised between the wall and the fire door when this is arranged in its closing position.