We present intermediate- and low-resolution optical spectroscopy (650-915 nm) of seven faint, very red objects ( 20>I>=17.8 , I-Z>= 0.5) discovered in a CCD-based IZ survey covering an area of 1 deg2 in the central region of the Pleiades open cluster. The observed spectra show that these objects are very cool dwarfs having spectral types in the range M6-M9. Five out of the seven objects can be considered Pleiades members on the basis of their radial velocities, H alpha emissions, and other gravity-sensitive atomic features like the Na I doublet at 818.3 and 819.5 nm. According to current evolutionary models, the masses of these new objects range from roughly 80 M_{{Jup}} for the hottest in the sample down to 45 M_{{Jup}} for Roque 4, the coolest and faintest confirmed member. These observations prove that the cloud fragmentation process extends well into the brown dwarf realm, suggesting a rise in the initial mass function below the substellar limit.