An automatic sliding-door power-drive assembly of the type comprising: a straight load-bearing longitudinal member which is arranged in a substantially horizontal position, above the sliding door to be driven; at least one shutter-supporting carriage which is slidingly mounted on the longitudinal member parallel to the longitudinal axis of the longitudinal member, and is structured to be fixed to the body of a sliding shutter of the sliding door; and carriage moving means which are fixed on the longitudinal member, and are designed so as to be able to move said at least one shutter-supporting carriage back and forth along the longitudinal member; the automatic sliding-door power-drive assembly being furthermore provided with a longitudinal casing which is fixed on the longitudinal member so as to hide said at least one shutter-supporting carriage and said carriage moving means, and with a presence sensor which is located outside the longitudinal casing and is capable of detecting the presence of things and/or people in the immediate nearby of the sliding door; the automatic sliding-door power-drive assembly being characterized in that said presence sensor is attached to the longitudinal member by means of a supporting bracket which is rigidly fixed directly on the load-bearing longitudinal member and is structured to bypass and extend around the longitudinal casing and keep the presence sensor stably suspended outside the longitudinal casing, wherein the supporting bracket is divided into an unmovable portion which is rigidly fixed to the load-bearing longitudinal member, and into a movable portion which directly supports the presence sensor and is movably mounted on the unmovable portion, wherein the load-bearing longitudinal member is provided with two protruding shelves (5, 6) which are arranged parallel and facing each other, so as to cantilevered extend from the wall one above the other; said at least one shutter-supporting carriage being movably mounted on the lower shelf of the longitudinal member; the carriage moving means being instead suspended below the upper shelf of the longitudinal member, wherein the longitudinal casing is fixed on the upper shelf of the longitudinal member and cantilevered extends downwards so as to be arranged straddling the lower shelf, and wherein the supporting bracket cantilevered extends downwards from the upper shelf of the longitudinal member so as to cantilevered project below the longitudinal casing, and is substantially hook-shaped so as to then extend again upwards while remaining locally substantially grazing the outer surface of the longitudinal casing; the presence sensor being fixed on the distal end of the supporting bracket. the automatic sliding-door power-drive assembly being furthermore provided with a longitudinal casing which is fixed on the longitudinal member so as to hide said at least one shutter-supporting carriage and said carriage moving means, and with a presence sensor which is located outside the longitudinal casing and is capable of detecting the presence of things and/or people in the immediate nearby of the sliding door; the automatic sliding-door power-drive assembly being characterized in that said presence sensor is attached to the longitudinal member by means of a supporting bracket which is rigidly fixed directly on the load-bearing longitudinal member and is structured to bypass and extend around the longitudinal casing and keep the presence sensor stably suspended outside the longitudinal casing, wherein the supporting bracket is divided into an unmovable portion which is rigidly fixed to the load-bearing longitudinal member, and into a movable portion which directly supports the presence sensor and is movably mounted on the unmovable portion, wherein the load-bearing longitudinal member is provided with two protruding shelves (5, 6) which are arranged parallel and facing each other, so as to cantilevered extend from the wall one above the other; said at least one shutter-supporting carriage being movably mounted on the lower shelf of the longitudinal member; the carriage moving means being instead suspended below the upper shelf of the longitudinal member, wherein the longitudinal casing is fixed on the upper shelf of the longitudinal member and cantilevered extends downwards so as to be arranged straddling the lower shelf, and wherein the supporting bracket cantilevered extends downwards from the upper shelf of the longitudinal member so as to cantilevered project below the longitudinal casing, and is substantially hook-shaped so as to then extend again upwards while remaining locally substantially grazing the outer surface of the longitudinal casing; the presence sensor being fixed on the distal end of the supporting bracket.