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Runaway-electron-preionized (REP) diffuse discharges are very attractive in many applications. It was found that, in the case of a cathode with a small radius of curvature and a flat anode, a diffuse corona arises within several hundreds of picoseconds at the early stage of the REP discharge. Both diffuse and spark discharges in dependence on the cathode-anode distance, type, and pressure of gas were observed. Images of REP diffuse and spark discharge plasma glowing in air, nitrogen, and its mixture with methane at different pressures are presented.