The present invention relates to new catalyst supports comprising nanofibers, a catalyst system comprising these supports as well as a process for preparing nanocomposites and the nanocomposites prepared. The invention especially concerns a supported catalyst system for polymerization of olefins, comprising a support made of fibers or a fleece of fibers, wherein the mean fiber diameter is less than 1000 nm, preferably less than 500 nm and the mean fiber length is more than 200,000 nm, preferably more than 500,000 nm and especially preferred more than 1,000,000 nm as well as a process for polymerizing olefinic systems in the presence of these catalyst systems and the resulting nanocomposites.