  The flat-spectrum radio quasar 3C 454.3 underwent an extraordinary 5-day
gamma-ray outburst in November 2010 where the daily flux measured with the
Fermi Large Area Telescope (LAT) at photon energies E>100 MeV reached (66+/-2)
x 10^-6 ph cm^-2 s^-1. This is a factor of 3 higher than its previous maximum
flux recorded in December 2009 and ~5 times brighter than the Vela pulsar,
which is normally the brightest source in the gamma-ray sky. The 3-hr peak flux
was (85+/-5) x 10^-6 ph cm^-2 s^-1, corresponding to an apparent isotropic
luminosity of 2.1+/-0.2 10^50 erg s^-1, the highest ever recorded for a blazar.
In this paper, we investigate the features of this exceptional event in the
gamma-ray band of the Fermi-LAT. In contrast to previous flares of the same
source observed with the Fermi-LAT, clear spectral changes are observed during
the flare.
