  Planetary systems form in gas-dust protoplanetary discs via the growth of
solid bodies. In this paper, we show that the most intriguing stage of such
growth --- namely, the transformation of 1-10 m boulders into kilometre-sized
planetesimals --- can be explained by a mechanism of gravitational instability.
The present work focused on the origin of self-gravitating clumps in which
planetesimal formation could take place. Our computer simulations demonstrated
that such clumps of gas and boulders formed due to the development of a
two-phase instability. This instability revealed a so-called 'mutual influence
effect' in the protoplanetary disc, where the dynamics of the system were
determined by the collisionless collective motion of a low-mass subdisc
composed of primary solids. We found that a 0.1 $c_s$ velocity dispersion in
the boulder subdisc was sufficient to cause the formation of self-gravitating
clumps of gas and boulders. In such regimes, the time needed for the formation
of the collapsing objects was less than the boulders' dissipation time in the
density waves of the medium.
