  We report results of ab initio constant-pressure molecular dynamics
simulations of sulfur compression leading to structural transition and
pressure-induced amorphization. Starting from the orthorhombic S-I phase
composed of S$_8$ ring molecules we find at room temperature and pressure of 20
GPa a transformation to monoclinic phase where half of the molecules develop a
different conformation. Upon further compression, the monoclinic phase
undergoes pressure-induced amorphization into an amorphous phase, in agreement
with experiments. We study the dynamics of the amorphization transition and
investigate the evolution of intra and intermolecular distances in the
monoclinic phase in order to provide a microscopic insight into the rings
disintegration process leading to amorphization. In the amorphous form we
examine the structural properties and discuss its relation to the
experimentally found amorphous form and to underlying crystal phases as well.
The amorphous form we find appears to correspond to the experimentally observed
low density amorphous form.
