An acceleration sensor for safety systems such as seat belt systems in motor vehicles, particularly so-called buckle-connected tighteners, comprises a sensor mass which is generally held in a position of rest but is capable of displacement in response to a predetermined critical acceleration to activate the safety system. In one embodiment, an inertial force sensor consists of a sensor lever pivoted to a part fixed in the vehicle, said sensor lever carries at its upwardly protruding free end an inertial mass, preferably in the form of an enlarged head, the sensor lever being held by a spring which urges said sensor lever toward its vertical stand-by position, and being connected by levers and/or links to a release mechanism for releasing a spring-loaded tie rod or the like or a blocking device. According to another feature, the sensor mass is journaled to a housing with the journal ends protruding from opposite housing walls and being received within hat-shaped caps which in turn are held in place by compression springs when the sensor mass is in its position of rest. The activation could be accomplished electrically by a microswitch.