We report on the effect of population inversion associated with the electron and hole injection in graphene pi- n structures at the temperatures 200K-300K. It is assumed that the recombination and energy relaxation of electrons and holes is associated primarily with the interband and intraband processes assisted by optical phonons. The dependences of the electron-hole and optical phonon effective temperatures on the applied voltage, the current-voltage characteristics, and the frequency-dependent dynamic conductivity are obtained. In particular, at low and moderate voltages the injection can lead to a pronounced cooling of the electron-hole plasma in the device i-section to the temperatures below the lattice temperature. At higher voltages, the current and electronhole and phonon temperature dependences on voltage exhibit the S-shape. At a certain values of the applied voltage the frequency-dependent dynamic conductivity can be negative in the terahertz range of frequencies. The electron-hole plasma cooling substantially reinforces the effect of negative dynamic conductivity and promotes the realization of terahertz lasing. It is demonstrated that the heating of optical phonon system hinders the realization of negative dynamic conductivity and terahertz lasing at the room temperatures.