The hydrodynamics and observational signatures of a relativistic shock break-out at the surface of Hypernova star are investigated. The characteristics of hydrodynamically accelerated external layers of star (energy spectrum of accelerated particles etc.) and the interaction of accelerated particles with the particles of circumstellar medium are considered. We analysed a gamma-ray flash as a result of a decay of neutral pions that are produced in inelastic p-p collisions. The parameters of the flash are calculated and conditions of its detection are estimated.