A fluid-conveying conduit of large cross section, formed by assembling on a laying surface a plurality of prefabricated longitudinal elements placed in sequence and comprising, in transverse section, a lower base and a vault having two lateral edges connected to the base by sealingly joining longitudinal members, which consist of two parallel metallic profiled members rigidly connected to one another by at least one crosspiece and fixed to a slab forming the base of the conduit, and the vault consists of at least one thin wall element and is sealingly fixed by its two lateral edges along two longitudinal profiled members. The invention applies especially to the construction of conduits for conveying fluid under pressure, for example for water conveyance, urban heating or penstocks.