  We study particle production at the preheating era in inflation models with
nonminimal coupling $\xi \phi^2R$ and quartic potential $\lambda \phi^4/4$ for
several cases: real scalar inflaton, complex scalar inflaton and Abelian Higgs
inflaton. We point out that the preheating proceeds much more violently than
previously thought. If the inflaton is a complex scalar, the phase degree of
freedom is violently produced at the first stage of preheating. If the inflaton
is a Higgs field, the longitudinal gauge boson production is similarly violent.
This is caused by a spike-like feature in the time dependence of the inflaton
field, which may be understood as a consequence of the short time scale during
which the effective potential or kinetic term changes suddenly. The produced
particles typically have very high momenta $k \lesssim
\sqrt{\lambda}M_\text{P}$. The production might be so strong that almost all
the energy of the inflaton is carried away within one oscillation for
$\xi^2\lambda \gtrsim {\mathcal O}(100)$. This may partly change the
conventional understandings of the (p)reheating after inflation with the
nonminimal coupling to gravity such as Higgs inflation. We also discuss the
possibility of unitarity violation at the preheating stage.
