A system for assembling a tower for a wind generator having a succession of subsections (Si). The system including: a plurality of lifting legs, a lifting structure vertically movable on the lifting legs, the lifting structure including a rigid lifting frame for holding the tower under assembly, the lifting frame including load transfer members and the lifting structure including self-climbing mechanisms interacting between the load transfer members and the lifting legs for elevating or lowering the lifting structure, and at least a first set of gripping arms movable relative to the lifting frame between a retracted configuration and a gripping configuration.