  We propose a model to explain the ultra-bright GeV gamma-ray flares observed
from the blazar 3C454.3. 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 3C454.3, 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.
