  We propose a straightforward and efficient mechanism for the high-energy
emission of relativistic astrophysical jets associated with an exchange of
interacting high-energy photons between the jet and the external environment.
Physical processes playing the main role in this mechanism are
electron-positron pair production by photons and the inverse Compton
scattering. This scenario has been studied analytically as well as with
numerical simulations demonstrating that a relativistic jet (with the Lorentz
factor larger than 3--4) moving through the sufficiently dense, soft radiation
field inevitably undergoes transformation into a luminous state. The process
has a supercritical character: the high-energy photons breed exponentially
being fed directly by the bulk kinetic energy of the jet. Eventually particles
feed back on the fluid dynamics and the jet partially decelerates. As a result,
a significant fraction (at least 20 per cent) of the jet kinetic energy is
converted into radiation mainly in the MeV -- GeV energy range. The mechanism
maybe responsible for the bulk of the emission of relativistic jets in active
galactic nuclei, microquasars and gamma-ray bursts.
