A catalyst system for the polymerization of olefins comprising (A) a catalyst component obtained by reacting a Mg(OR1)(OR2) compound, in which R1 and R2 are identical or different and are each an alkyl radical having 1 to 10 carbon atoms, with a tetravalent transition metal compound having at least a Metal-halogen bond, used in amounts such that the molar ratio Met-al/Mg is from 0.05 to 10; (B) an aluminum alkyl compound and (C) a linear or branched halogenated alkyl compound.