Abstract: A process is disclosed for the polymerization of olefins, using (i) a catalyst and (ii) an organometallic activator from Groups I, II or III of the Periodic Table at temperatures between 20.degree. and 250.degree. C., and pressure up to 200 bar, in solution, or in suspension in an inert liquid hydrocarbon, at temperatures between 160.degree. and 350.degree. C., under pressures of between 400 and 3000 bar, in a continuous manner in a reactor where the mean residence time of the catalyst is between 1 and 150 seconds. The catalyst comprises at least one titanium compound, at least one halogen-containing organoaluminum compound and at least one inorganic magnesium compound in suspension in at least one .alpha.,.omega.-dihaloalkane. The titanium compound is essentially a titanium (III) compound, the total content of titanium (II) and titanium (IV) being less than or equal to 15% of the total content of titanium.