Particle detectors of the European SEVAN network located on mountain heights in Aragats (Armenia), Lomnicky Stit (Slovakia) and Musala (Bulgaria) are well suited for the detection of thunderstorm ground enhancements (TGEs, enhanced fluxes of electrons, gamma rays, neutrons). The modulation of charged particles flux by the electric field of the thundercloud results in a sizable change in the count rate of detectors, which measure fluxes of electrons, gamma rays, and high energy muons in the near-vertical and near-horizontal directions. The relation between electric field strength and changes of particle flux count rates is nonlinear and depends on many unknown parameters of atmospheric electric field and meteorological conditions. Nonetheless, employing extreme TGEs as a manifestation of the strong electric field in the thundercloud and by measuring fluxes of three species of secondary cosmic rays (electrons, gamma rays, and muons) by SEVAN detectors located at altitudes of ≈ 3 km we study the extreme strength of the atmospheric electric field. With the simulation of particle traversal through the electric field with CORSIKA code, we derive a maximum potential difference in the thunderous atmosphere to be ≈ 500 MV.