A process for the production of polyolefins is disclosed, which is carried out in the presence of catalysts prepared according to the invention. The catalysts comprise a carrier resulting from the reaction of anhydrous magnesium halide, silicon tetrachloride and an alcohol represented by the general formulaROHwhere R is an organic group of 1 to 20 carbon atoms, and a titanium and/or a vanadium compound supported on said carrier. Such catalysts are activated by an organoaluminum and/or an organozinc compound when in use.