Abstract: A system of grandstand with foldable bleachers, comprises an under-frame (1) mounted on wheels (2) and supporting a central horizontal floor (3). The floor (3) is provided with upright elements (8, 9), upon which are externally mounted, around horizontal axis (11, 12), two pivotable longitudinal lateral floors (15, 16), which can pivot between a first configuration in which they are folded one towards the other to form a space assembly meeting the legal dimension requirements which apply to trailers, and which can be hauled by a vehicle comprising the underframe (1) and wheels (2, and a second configuration in which they are unfolded externally at different levels to form a generally plane and oblique structure upon which seats (17) and (18) are mounted as bleachers forming an autonomous grandstand. An apparatus is also provided for folding and unfolding said floors in a unique manual operation.