  It is commonly assumed that photocurrent in two-dimensional systems with
centrosymmetric lattice is generated at structural inhomogenities, such as p-n
junctions. Here, we study an alternative mechanism of photocurrent generation
associated with inhomogenity of the driving electromagnetic field, termed as
'plasmonic drag'. It is associated with direct momentum transfer from field to
conduction electrons, and can be characterized by a non-local non-linear
conductivity $\sigma^{(2)}({\bf q},\omega)$. By constructing a classical
kinetic model of non-linear conductivity with full account of non-locality, we
show that it is resonantly enhanced for wave phase velocity coinciding with
electron Fermi velocity. The enhancement is interpreted as phase locking
between electrons and the wave. We discuss a possible experiment where
non-uniform field is created by a propagating graphene plasmon, and find an
upper limit of the current responsivity vs plasmon velocity. This limit is set
by a competition between resonantly growing $\sigma^{(2)}({\bf q},\omega)$ and
diverging kinetic energy of electrons as the wave velocity approaches Fermi
velocity.
