The invention relates to a system for externally holding a catalyst bed in a radial flow catalytic reactor, comprising a plurality of hollow longitudinal duct elements (2) regularly distributed in a peripheral ring around a central space (3) containing the catalyst bed and having a front filtering wall (5) facing the catalyst for the radial intake of a fluid to be treated in the catalyst bed (3). Each of the duct elements (2) is laterally defined by two lateral walls (6) radially oriented in the reactor (1) and comprising an assembling means for assembly with the respectively adjacent elements (2′) of the male member in female member (7, 8) type.