Almost half of primeval galaxies show P-Cygni type profiles in the Lyα emission line. The main underlying mechanism for the profile formation in these systems is thought to be the frequency re-distribution of the line photons in expanding scattering media surrounding the emission source. A Monte Carlo code is developed to investigate the Lyα line transfer in an optically thick and moving medium with a careful consideration of the scattering in the damping wings. Typical column densities and expansion velocities of neutral hydrogen investigated in this study are NH1 ~10 17-20 cm -2 and ΔV ~ 100 km s-1. We investigate the dependence of the emergent profiles on the kinematics and on the column density. Our numerical results are applied to show that the damped Lyα absorbers may possess an expanding H I supershell with bulk flow of ~ 200 km s-l and H I column density NH1 ~ 10 19 cm -2. We briefly discuss the observational implications.