  The formation of primordial black holes in the early universe in the
Brans-Dicke scalar-tensor theory of gravity is investigated. Corrections to the
threshold value of density perturbations are found. Above the threshold, the
gravitational collapse occurs after the cosmological horizon crossing. The
corrections depend in a certain way on the evolving scalar field. They affect
the probability of primordial black holes formation, and can lead to their
clustering at large scales if the scalar field is inhomogeneous. The formation
of the clusters, in turn, increases the probability of black holes merge and
the corresponding rate of gravitational wave bursts. The clusters can provide a
significant contribution to the LIGO/Virgo gravitational wave events, if part
of the observed events are associated with primordial black holes.
