  The emission mechanism of the gamma-ray bursts (GRBs) is still a matter of
debates. The standard synchrotron energy spectrum of cooling electrons F_E ~
E^{-1/2} is much too soft to account for the majority of the observed spectral
slopes. An alternative in the form of quasi-thermal Comptonization in a high
compactness source has difficulties in reproducing the peak of the observed
photon distribution below a few hundred keV. We show here that for typical
parameters expected in the GRB ejecta the observed spectra in the 20-1000 keV
BATSE energy range can be produced by inverse Compton scattering of the
synchrotron radiation in a partially self-absorbed regime. If the particles are
continuously accelerated/heated over the life-time of a source rather than
being instantly injected, a prominent peak develops in their distribution at a
Lorentz factor gamma ~ 30-100, where synchrotron and inverse-Compton losses are
balanced by acceleration and heating due to synchrotron self-absorption. The
synchrotron peak should be observed at 10-100 eV, while the self-absorbed
low-energy tail with F_E ~ E^2 can produce the prompt optical emission (like in
the case of GRB 990123). The first Compton scattering radiation by nearly
monoenergetic electrons can then be as hard as F_E ~ E^1 reproducing the
hardness of most of the observed GRB spectra. The second Compton peak should be
observed in the high energy gamma-ray band, possibly being responsible for the
emission detected by EGRET in GRB 941017. A significant electron-positron pair
production reduces the available energy per particle, moving the spectral peaks
to lower energies as the burst progresses. The regime is very robust, operates
in a broad range of parameter space and can explain most of the observed GRB
spectra and their temporal evolution.
