  We study the interaction-induced quantum correction
\delta\sigma_{\alpha\beta} to the conductivity tensor of electrons in two
dimensions for arbitrary T\tau (where T is the temperature and \tau the
transport scattering time), magnetic field, and type of disorder. A general
theory is developed, allowing us to express \delta\sigma_{\alpha\beta} in terms
of classical propagators (``ballistic diffusons''). The formalism is used to
calculate the interaction contribution to the longitudinal and the Hall
resistivities in a transverse magnetic field in the whole range of temperature
from the diffusive (T\tau << 1) to the ballistic (T\tau > 1) regime, both in
smooth disorder and in the presence of short-range scatterers. Further, we
apply the formalism to anisotropic systems and demonstrate that the interaction
induces novel quantum oscillations in the resistivity of lateral superlattices.
