A catalytic system characterized in that it comprises: a first catalyst, having a hydrogenating function, consisting of solid particles of which at least 95% by volume having an equivalent diameter smaller than 20 μm, containing one or more sulfides of metals of group VI and/or VIII B, possibly prepared starting from an oil-soluble precursor of the same; and a second catalyst, having a cracking function, consisting of solid particles of which at least 90% by volume having an equivalent diameter larger than 5 μm and smaller than 5 mm, containing an amorphous silico-aluminate and/or a crystalline silico-aluminate and/or an alumina, the equivalent average diameter of the solid particles of the second catalyst being greater than the equivalent average diameter of the solid particles of the first catalyst. Said catalytic system can be used in a process for the hydroconversion of heavy oils which comprises sending the heavy oil to a hydroprocessing step carried out in one or more slurry reactors, in the presence of hydrogen or a mixture of hydrogen and H2S, obtaining a stream of products in vapor or liquid-vapor phase, and extracting at the bottom, in continuous or discontinuous, a liquid stream containing non-converted products together with the two catalysts of said catalytic system, said liquid stream extracted then being separated into a clarified primary stream containing the first catalyst, which is at least partially recycled to the hydroprocessing step, and a stream rich in the second catalyst, which is regenerated in a regeneration step and at least partially recycled to the hydroprocessing step.