A polymerization process for preparing heterophasic propylene copolymers in two or more gas-phase reactors connected in series, in the presence of a polymerization catalyst comprising a catalytic component based on a titanium compound supported on magnesium halide, the process comprising the following steps: - contacting said catalytic component with an organo-Aluminum compound at a temperature from 5°C to 30°C and a weight ratio propylene/(catalytic component) ranging from 0 to 2.0; - prepolymerizing the catalyst from A) with propylene, optionally in the presence of an inert hydrocarbon solvent; - polymerizing propylene, optionally with an another -olefin comonomer in an amount lower than 15% by weight, to prepare a semicrystalline polymer component; - successively copolymerizing two or more alpha-olefin comonomers CSUB2/SUB-CSUB10/SUB to prepare one or more olefin copolymers having a solubility in xylene higher than 15% by weight; the process being characterized in that the bulk density of the semi-crystalline component of step C) is adjusted at a value lower than 0.40 g/cmSUP3/SUP.
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