From drug delivery1, 2 to chemical and biological catalysis3 and cosmetics4, the need for efficient fabrication pathways for particles over a wide range of sizes, from a variety of materials, and in many different structures has been well established5. Here we harness the inherent scalability of fibre production6 and an in-fibre Plateau–Rayleigh capillary instability7 for the fabrication of uniformly sized, structured spherical particles spanning an exceptionally wide range of sizes: from 2mm down to 20nm. Thermal processing of a multimaterial fibre8 controllably induces the instability9, resulting in a well-ordered, oriented emulsion10 in three dimensions. The fibre core and cladding correspond to the dispersed and continuous phases, respectively, and are both frozen in situ on cooling, after which the particles are released when needed. By arranging a variety of structures and materials in a macroscopic scaled-up model of the fibre, we produce composite, structured, spherical particles, such as core–shell particles, two-compartment ‘Janus’ particles11, and multi-sectioned ‘beach ball’ particles. Moreover, producing fibres with a high density of cores allows for an unprecedented level of parallelization. In principle, 108 50-nm cores may be embedded in metres-long, 1-mm-diameter fibre, which can be induced to break up simultaneously throughout its length, into uniformly sized, structured spheres.