A turbomachine fan rotor including a disk carrying blades with roots that are engaged in axial grooves of the disk, a spacer being interposed between the bottom of each groove and the corresponding blade root, the spacer including at its upstream end a nib for retaining the blade root, the nib including a downstream abutment face against which the upstream end of the blade root bears in event of a violent force being applied thereto, the abutment face being oblique relative to the axis of rotation of the rotor.