  We study the dynamics of a supersonic molecular beam in a low-finesse optical
cavity and demonstrate that most molecules in the beam can be decelerated to
zero central velocity by the intracavity optical field in a process analogous
to electrostatic Stark deceleration. We show that the rapid switching of the
optical field for slowing the molecules is automatically generated by the
cavity-induced dynamics. We further show that $\sim1\%$ of the molecules can be
optically trapped at a few millikelvin in the same cavity.
