  We predict the general feasibility and demonstrate the specific design of the
THz laser operating between Landau levels in graphene placed on a polar
substrate in a magnetic field of order 1 T. Steady state operation under a
continuous wave optical pumping is possible due to an interplay between Auger
and surface-phonon mediated relaxation of carriers. The scheme is scalable to
other materials with massless Dirac fermions, for example surface states in 3D
topological insulators such as Bi$_2$Se$_3$ or Bi$_2$Te$_3$.
