Abstract: An antitheft system installed on a motor vehicle has an electronic control unit connected to a localization information detector and to a telephone apparatus, so that the control unit can produce a voice message and then transmit it directly to the police and/or to a security organization by means of the telephone apparatus, when a plurality of sensors detect certain conditions such as the theft of the vehicle. The message may also be sent by the system to a person authorized to use the motor vehicle by means of a transmitter-receiver radio and a portable transceiver radio. Voice messages are contained in a first mass memory unit, localization information derived from a second mass memory unit, the message selected on the basis of satellite signals received and decoded by the system.