We report on the lasing action of atmospheric air pumped by an 800 nm femtosecond laser pulse with peak power up to 4 TW. Lasing emission at 428 nm increases rapidly over a small range of pump laser power, followed by saturation above ∼1.5 TW. The maximum lasing pulse energy is measured at 2.6 μJ corresponding to an emission power in the MW range, while a maximum conversion efficiency of 3.5×10(-5) is measured at moderate pump pulse energy. The optical gain inside the filament plasma is estimated to be in excess of 0.7/cm. Lasing emission shows a doughnut profile, reflecting the spatial distribution of the pump-generated white-light continuum that acts as a seed for the lasing. We attribute the pronounced saturation to the defocusing of the seed in the plasma amplifying region and to the saturation of the seed intensity.