We report on a fundamentally new radiation phenomenon in nanostructures: emission of spatially coherent and spectrally narrow-band light driven by localized injection of free electrons into a planar plasmonic metamaterial. In 'coherent' metamaterials, where interactions among the meta-molecules are strong, an electron beam can generate a synchronized, in-phase response across a large number of meta-molecules leading to threshold-free emission of spatially coherent light propagating perpendicular to the metamaterial.In a laser coherence is derived from the bosonic statistics of photons, the resonant properties of the laser cavity and the collective nature of stimulated emission in the gain medium, and a threshold level of input energy is required to maintain the gain and overcome losses. Here we show experimentally that a scalable low-divergence, threshold-free optical source can be constructed on the basis of a collectively oscillating optical nano-antenna array formed by the metamaterial wherein coherence is provided not by the feed (as in conventional microwave antenna arrays) but via the strong mutual interactions among the nano-antennas.