The present invention refers to an improved electro-mechanical lock wherein a connecting rod is articulated at its ends to the bolt and to a lever or crank, the latter being able to rotate about the axis of articulation of the connecting rod, with the special feature that the basic assembly formed by the bolt-connecting rod is constantly impelled by the action of a tension spring which has one of its ends anchored to the end of the articulation shaft of the connecting rod and of the crank. Provision has been made for a counterfort on one lateral of the general casing or housing, on which counterfort there is an offset intended to house the head to the above-mentioned shaft in the last place, in the open position, while in the closed position the head of the shaft is blocked and with it the whole connecting rod-bolt assembly by means of a ratchet which covers the entrance to the cavity or offset made in the counterfort, the ratchet being displaced from the closed position by means of a rigid rod moved by the action of an electromagnet, so that when the said action ceases the ratchet tends to recover the closed position by means of an antagonistic spring placed between the rod and the casing.