An interlocking masonry block system for the construction of load bearing and non-load bearing walls includes a plurality of generally rectangular shaped building blocks wherein each block comprises a pair of opposite longitudinally extending sidewalls each of which includes an interior rectangular shaped vertical rib. The block also includes a central section disposed between the sidewalls in an abutting relationship therewith. Further, the central section defines one or two diagonally oriented rectangular vertical passageway extending through the central section at an angle of between about 30° and 45° and preferably at an angle of about 38°. The central section also includes one to three vertical extending recesses corresponding to and mating with the vertical ribs on each side thereof. The central section and the pair of sidewalls also define up to four rectangular shaped vertical passageways for utility pipes and/or electrical conduits.