Detecting or measuring apparatus comprising an activation member, as a switch and the like, which can be activated by contact, for the activation of a warning system, and an anti-tamper device, comprising: a support, provided with longitudinal guide means and transversal guide means, arranged according to substantially mutually orthogonal directions, a sliding transverse member, slidably engaged with said transversal guide means and movable between a first central neutral position and two first lateral operating positions, respectively opposite said central neutral position, said sliding transverse member being provided with at least two elements (11, 11'), which react to a magnetic field, which they can be subjected to, so as to be subjected to attraction or repulsion and arranged in correspondence of opposite ends of said sliding transverse member, a longitudinal sliding member, slidably movable on said longitudinal guide means and provided with a projection, said longitudinal member being movable between a second neutral position and a second operating position, in which said projection contacts said activation member, movable connection means (12, 16) for the mutual movable connection of said sliding transverse member and said longitudinal sliding member, said movable connection means (12, 16) being configured in such a way that, when the sliding transverse member is arranged in said first central neutral position, the longitudinal sliding member is arranged in said second neutral position, and, when the sliding transverse member is arranged in one of said two first lateral operating positions, the longitudinal sliding member is arranged in said second operating position. a support, provided with longitudinal guide means and transversal guide means, arranged according to substantially mutually orthogonal directions, a sliding transverse member, slidably engaged with said transversal guide means and movable between a first central neutral position and two first lateral operating positions, respectively opposite said central neutral position, said sliding transverse member being provided with at least two elements (11, 11'), which react to a magnetic field, which they can be subjected to, so as to be subjected to attraction or repulsion and arranged in correspondence of opposite ends of said sliding transverse member, a longitudinal sliding member, slidably movable on said longitudinal guide means and provided with a projection, said longitudinal member being movable between a second neutral position and a second operating position, in which said projection contacts said activation member, movable connection means (12, 16) for the mutual movable connection of said sliding transverse member and said longitudinal sliding member, said movable connection means (12, 16) being configured in such a way that, when the sliding transverse member is arranged in said first central neutral position, the longitudinal sliding member is arranged in said second neutral position, and, when the sliding transverse member is arranged in one of said two first lateral operating positions, the longitudinal sliding member is arranged in said second operating position.