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In this work we show that the strong interaction between a surface plasmon and a set of localized independent emitters can lead to the formation of a macroscopic extended state, formed by the superposition of plasmon and excitations on a large number of molecules. The sample consists of an aggregated dye layer deposited on silver supporting surface plasmons. The aggregated dye layer is constituted by an ensemble of short length independent chains whose emission is incoherent without metal. The diffusion and the spatial coherence of the emission of J-aggregated dyes on silver have been investigated with Young-type interferometric experiments, evidencing an in-phase emission of localized emitters separated by several microns. The extension of this coherent state over a large number of molecules could lead to energy transfer on a micrometer scale mediated by the plasmon.