  We show that sharply defined topological quantum phase transitions are not
limited to states of matter with gapped electronic spectra. Such transitions
may also occur between two gapless metallic states both with extended Fermi
surfaces. The transition is characterized by a discontinuous, but not
quantized, jump in an off-diagonal transport coefficient. Its sharpness is
protected by a symmetry, such as e.g. particle-hole, which remains unbroken
across the transition. We present a simple model of this phenomenon, based on
2D $p+ip$ superconductor with an applied supercurrent, and discuss its
geometrical interpretation.
