The safety belt pre-tension assembly comprises a support 14, a safety belt buckle 10 mounted on the support for axial displacement relative to the support, a housing 40, a winding reel 44 mounted within the housing and a helical power spring 52 associated with the reel to cause rotation of the winding reel. A flexible cable 34 has one end connected to the winding reel, so that the flexible cable will be wound thereon upon operation of the power spring. The other end of the flexible cable 34 is connected to the buckle 10 to move the buckle axially in a first direction effective to tension a safety belt held by the buckle. Actuation means 54-70 are provided to trigger actuation of the spring 52 upon the actuation means being subjected to a deceleration greater than the predetermined value. Locking means 26-32 are provided to allow the buckle to move in the first direction only, but not in the opposite direction.