Disclosed is a method of: providing a mixture of a polymer or a resin and a transition metal compound, producing a fiber from the mixture, and heating the fiber under conditions effective to form a carbon nanotube-containing carbonaceous fiber. The polymer or resin is an aromatic polymer or a precursor thereof and the mixture is a neat mixture or is combined with a solvent. Also disclosed are a carbonaceous fiber or carbonaceous nanofiber sheet having at least 15 wt. % carbon nanotubes, a fiber or nanofiber sheet having the a polymer or a resin and the transition metal compound, and a fiber or nanofiber sheet having an aromatic polymer and metal nanoparticles.