  We propose a model to explain the ultra-bright GeV gamma-ray flares observed
from the blazar \c454. The model is based on the concept of a relativistic jet
interacting with compact gas condensations produced when a star (red giant)
crosses the jet close to the central black hole. The study includes an
analytical treatment of the evolution of the envelop lost by the star within
the jet, and calculations of the related high-energy radiation.
  The model readily explains the day-long, variable on timescales of hours, GeV
gamma-ray flare from \c454, observed during November 2010 on top of a
weeks-long plateau. In the proposed scenario, the plateau state is caused by a
strong wind generated by the heating of the star atmosphere by nonthermal
particles accelerated at the jet-star interaction region. The flare itself
could be produced by a few clouds of matter lost by the red giant after the
initial impact of the jet. In the framework of the proposed scenario, the
observations constrain the key model parameters of the source, including the
mass of the central black hole: $M_{\rm BH}\simeq 10^9 M_{\odot}$, the total
jet power: $L_{\rm j}\simeq 10^{48}\,\rm erg\,s^{-1}$, and the Doppler factor
of the gamma-ray emitting clouds, $\delta\simeq 20$. Whereas we do not specify
the particle acceleration mechanisms, the potential gamma-ray production
processes are discussed and compared in the context of the proposed model. We
argue that synchrotron radiation of protons has certain advantages compared to
other radiation channels of directly accelerated electrons.
