We demonstrate the efficient generation of Raman sidebands in a medium coherently prepared in a dark state by continuous-wave low-intensity laser radiation. Our experiment is performed in sodium vapor excited in Λ configuration on the D1 line by two laser fields of resonant frequencies ω1 and ω2, and probed by a third field ω3. First-order sidebands for frequencies ω1, ω2 and up to the third-order sidebands for frequency ω3 are observed. The generation starts at a power as low as 10 microwatt for each input field. Dependencies of the intensities of both input and generated waves on the frequency difference (ω1 − ω2), on the frequency ω3 and on the optical density are investigated.42.50. Gy, 32.80.Qk, 42.50.Hz