  The dynamic transition between the ordered flow and the plastic flow is
studied for a two-dimensional driven vortex lattice, in the presence of sharp
and dense pinning centers, from numerical simulations. For this system, which
does not show smectic ordering, the lattice exhibits a first order transition
from a crystal to a liquid, shortly followed by the dynamical transition to the
plastic flow. The resistivity provides a critical order parameter for the
latter, and critical exponents are determined in analogy with a percolation
transition. At the boundary between a pinned region and an unpinned one, an
anomalous layer is observed, where the vortices are more strongly pinned than
in the bulk.
