A process and reactive prepolymer composition for producing a part made of a thermoplastic composite material by molding in a closed mold, where the material includes reinforcing fibers and a polyamide thermoplastic matrix impregnating the fibers having the steps of preparing the reactive prepolymer precursor, injecting the reactive prepolymer precursor in the molten state into the closed mold containing the fibers, thereby impregnating the fibers with the reactive precursor mixture, bulk polymerizing the reactive prepolymer precursor in situ, and demolding the molded part produced.