Process for the polymerization of olefins, according to which at least one olefin is placed in contact with a catalytic system comprising: a) a solid catalytic complex based on magnesium, transition metal and halogen, the said catalytic complex being prepared by reacting, in a first step, at least one magnesium compound with at least one compound of a transition metal from group IVB or VB of the Periodic Table, until a liquid complex is obtained, and, in a subsequent step, by precipitating the said liquid complex using a halogen-containing organoaluminium compound of general formula AlRnX3-n in which R is a hydrocarbon radical, X is a halogen and n is less than 3, in order to collect a solid catalytic complex, and b) an organometallic compound of a metal from groups IA, IIA, IIB, IIIA and IVA of the Periodic Table, c) at least one electron donor used after the first step in the preparation of the solid catalytic complex leading to the production of a liquid complex.