Experimental and theoretical investigation of the coherent propagation and amplification of polychromatic broadband laser pulses in optically dense resonant media without population inversion was carried out for the cases of plasma in a Ne glow discharge and a pulse discharge in Hg-Ar mixture. High density of Ne and Hg atoms in the metastable state (about 10¹² cm^-3), which was chosen to be the ground state of the two-level system under consideration, leads to the collective behavior of the atomic system in resonant electromagnetic field. The broadband probe field attenuation and the sideband amplification of the probe under the action of the coherent pump were observed. The amplification dependence on the pump intensity was obtained. The model of the parametrically enhanced collective interactions was developed and comparison to the experimental data was made.