The system comprises a plurality of modular blocks (10, 14) of nonmagnetic material, to be assembled to create compositions imitating, for example, a building, a doll, an animal, a vehicle or any other fancying assembly; a framework for connecting the blocks, comprises a plurality of magnetic frame members (11, 12) removably disposable into corresponding seatings (13, 17) in each modular block (10, 14), or between adjacent blocks of the composition. The modular blocks (11, 12) and the magnetic frame members of framework have shoulder surfaces (18, 19) mutually engageable in the assembled condition of the assembly.