The one-dimensional extended Peierls-Hubbard model is studied at several band fillings using the density-matrix renormalization-group method. Results show that the ground state evolves from a Mott-Peierls insulator, with a correlation gap at half-filling, to a soliton lattice with a small band gap away from half-filling. It is also confirmed that the ground state of the Peierls-Hubbard model undergoes a transition to a metallic state at finite doping. These results show that electronic correlation effects should be taken into account in theoretical studies of doped polyacetylene. They also show that a Mott-Peierls theory could explain the insulator-metal transition observed in this material.