The present invention relates to a process for homopolymerization of ethylene or copolymerization of ethylene with alpha-olefins by contacting ethylene or ethylene and alpha-olefin with a catalyst composition comprising: (a) a solid catalyst precursor comprising at least one vanadium compound, at least one magnesium compound and a polymeric material or a solid catalyst precursor comprising at least one vanadium compound, at least one further transition metal compound and/or at least one alcohol, at least one magnesium compound and a polymeric material; and (b) a cocatalyst comprising an aluminum compound.