  The blazar 3C454.3 exhibited a strong flare seen in gamma-rays, X-rays, and
optical/NIR bands during 3--12 December 2009. Emission in the V and J bands
rose more gradually than did the gamma-rays and soft X-rays, though all peaked
at nearly the same time. Optical polarization measurements showed dramatic
changes during the flare, with a strong anti-correlation between optical flux
and degree of polarization (which rose from ~ 3% to ~ 20%) during the declining
phase of the flare. The flare was accompanied by large rapid swings in
polarization angle of ~ 170 degree. This combination of behaviors appear to be
unique. We have cm-band radio data during the same period but they show no
correlation with variations at higher frequencies. Such peculiar behavior may
be explained using jet models incorporating fully relativistic effects with a
dominant source region moving along a helical path or by a shock-in-jet model
incorporating three-dimensional radiation transfer if there is a dominant
helical magnetic field. We find that spectral energy distributions at different
times during the flare can be fit using modified one-zone models where only the
magnetic field strength and particle break frequencies and normalizations need
change. An optical spectrum taken at nearly the same time provides an estimate
for the central black hole mass of ~ 2.3 * 10^9 M_sun. We also consider two
weaker flares seen during the $\sim 200$ d span over which multi-band data are
available. In one of them, the V and J bands appear to lead the $\gamma$-ray
and X-ray bands by a few days; in the other, all variations are simultaneous.
