Topological phase transitions in two-dimensional systems with internal symmetries

Possible generalizations of the topological (or Berezinskii-Kosterlitz-Thouless) phase transition on multicomponent 2D systems with nontrivial vector homotopic group pi_1 are considered. Relations between Ginzburg-Landau like theories, non-linear sigma-models on maximal Cartan subgroups of simple compact Lie groups and generalized sine-Gordon type theories are discussed. D-dimensional non-linear sigma-model admitting topological excitations with logarithmic energies are constructed.