ABSTRACT Numerical simulations of cosmological structure formation show that the universe's most massive clusters, and the galaxies living in those clusters, assemble rapidly at early times ( ). While more than 20 proto-clusters have been observed at based on associations of 5-40 galaxies around rare sources, the observational evidence for rapid cluster formation is weak. Here we report observations of an asymmetric filamentary structure at z = 2.47 containing 7 starbursting, submillimeter-luminous galaxies and 5 additional active galactic nuclei (AGNs) within a comoving volume of 15,000 Mpc3. As the expected lifetime of both the luminous AGN and starburst phase of a galaxy is ∼100 Myr, we conclude that these sources were likely triggered in rapid succession by environmental factors or, alternatively, the duration of these cosmologically rare phenomena is much longer than prior direct measurements suggest. The stellar mass already built up in the structure is ∼1012 and we estimate that the cluster mass will exceed that of the Coma supercluster at . The filamentary structure is in line with hierarchical growth simulations that predict that the peak of cluster activity occurs rapidly at .