An olefin polymer composition comprising (by weight, unless otherwise specified): A) 60-85% of a crystalline propylene homopolymer or a crystalline copolymer of propylene containing 3% or less of ethylene or C4-C10 α-olefin(s) or of combinations thereof, said homopolymer or copolymer having a Polydispersity Index (P.I.) value of from 4.5-6 and a content of isotactic pentads (mmmm), measured by 13C NMR on the fraction insoluble in xylene at 25° C., higher than 96%; B) 15-40% of a partially amorphous copolymer of ethylene containing from 35% to 70% of propylene or C4-C10 α-olefin(s) or of combinations thereof, and optionally minor proportions of a diene. The said olefin polymer composition exhibits a value of elongation at break ranging from 150 to 600% according to ISO method 527.