A catalytic system usable for the copolymerization of at least one conjugated diene and at least one monoolefin, a process for preparing this catalytic system, a process for preparing a copolymer of a conjugated diene and at least one monoolefin using said catalytic system, and said copolymer are described. This catalytic system includes: (i) an organometallic complex represented by the following formula: {[P(Cp)(Fl)Ln(X)(L.sub.x)}.sub.p (1) where Ln represents a lanthanide atom to which is attached a ligand molecule comprising cyclopentadienyl Cp and fluorenyl Fl groups linked to one another by a bridge P of the formula MR.sup.1R.sup.2, M is an element from column IVa of Mendeleev's periodic table and R.sup.1 and R.sup.2 each represent alkyl groups of 1 to 20 carbon atoms or cycloalkyl or phenyl groups of 6 to 20 carbon atoms, X represents a halogen atom, L represents an optional complexing molecule, such as an ether, and optionally a substantially less complexing molecule, such as toluene, p is a natural integer greater than or equal to 1 and x is greater than or equal to 0, and (ii) a co-catalyst selected from alkylmagnesiums, alkyllithiums, alkylaluminums, Grignard reagents and mixtures of these constituents.