Nano-Sized light emitting devices have a wide range of potential applications, from medicine to all-optical computing. Surface plasmon amplification by stimulated emission of radiation (SPASER) have recently attracted considerable attention providing the smallest reliable lasers. Plasmonic assisted lasing has been demonstrated in a variety of geometries, embedded in or embedding a gain media. Despite these efforts, the physics underlying a realistic SPASER remains to date only drafted, because of the controversial and the limited number of experimental results. Here we demonstrate unambiguous coherent emission from polyhedral silver nano-particles dispersed in liquid gain media evidenced clearly by the narrowing of the spectral linewidth up to 3-5 nm, the several competing SPASER modes and the nonlinear effects as emission saturation and energy dependent spectral shifts. These novel results open the way to a variety of applications, as the emission of ultrashort optical pulses from nano-size lasers.