A polyolefin composition, particularly fit for the production of extrusion blow molded articles, comprising (a) a propylene-ethylene copolymer having a content of units deriving from ethylene of 4.0% by weight or higher, and (b) a nucleating agent and having a crystallization temperature (Tc) higher than 117° C. can be obtained by copolymerizing propylene and ethylene in the presence of a catalyst system obtained by contacting a solid catalyst component comprising a magnesium halide, a titanium compound having at least a Ti-halogen bond and at least two electron donor compounds one of which being present in an amount from 40 to 90% by mol with respect to the total amount of donors and selected from succinates and the other selected from 1,3 diethers, an aluminum hydrocarbyl compound, and optionally an external electron donor compound.