The present invention is related to single and/or multiple-wall carbon nanotubes which may contain interstitial metals obtainable by a preparation process. The process includes a catalytic step using a catalytic system which includes a catalyst and a support. The support comprises hydroxides and/or carbonates or mixtures thereof with or without metal oxides. The present invention is also related to carbon fibers obtainable by said preparation process. The present invention also pertains in particular to said catalytic system and to said preparation process. Another aspect concerns the use of the nanotubes and of the catalytic system according to the invention.