The system comprising a base (2) having attachment means (3) which are movable between a retracted position for fitting/dismantling the seat and an active position for attachment of the seat, by an operating lever (4) which is movable between a retracted fitting/dismantling position and an active position for blocking of the attachment means and which has means (6) for articulation with the base (2) and disengageable means (8) for locking the lever in the active position, is characterized in that the locking means comprise means in the form of a catch (9) which are carried by the lever or the base and are suitable for cooperation, in the active position, with holding means (10) of the base or of the lever in order to lock the lever (4) in the active position, the means in the form of a catch (9) being resiliently deformable in order to allow them to pass over the holding means (10) and engage in the locking position and to be freed therefrom under the action of rotary operating means (18).