A multistage process for the homo or copolymerization of ethylene carried out in the presence of a catalyst system comprising at least one polymerization catalyst (A) based on a late transition metal component having a tridentate ligand which bears at least two ortho.ortho-disubstituted aryl radicals, at least one Ziegler polymerization catalyst (B), and at least a Lewis acid activating compound (C), wherein the molar ratio of the metal of the activating compound (C) to the late transition metal of catalyst (A) is different in the two polymerizations steps. Catalysts (A) and (B) may be selectively controlled by varying the amount of activating compound (C).