A gas-phase process for the homopolymerization or copolymerization of propylene with other olefins, including carrying out the polymerization in the presence of a catalyst system made from or containing: (a) a solid catalyst component made from or containing Mg, Ti, halogen, an electron donor selected from 1.3-diethers and an olefin polymer in a specific amount; (b) an aluminum alkyl compound and (c) an external electron donor (ED) compound, wherein components (b) and (c) being employed in amounts such that the Al/(ED) molar ratio ranges from about 2 to about 200.