  The quasar-type blazar 3C 454.3 underwent a phase of high activity in summer
and autumn 2007, which was intensively monitored in the radio-to-optical bands
by the Whole Earth Blazar Telescope (WEBT). The gamma-ray satellite AGILE
detected this source first in late July, and then in November-December 2007. In
this letter we present the multifrequency data collected by the WEBT and
collaborators during the second AGILE observing period, complemented by a few
contemporaneous data from UVOT onboard the Swift satellite. The aim is to trace
in detail the behaviour of the synchrotron emission from the blazar jet, and to
investigate the contribution from the thermal emission component. Optical data
from about twenty telescopes have been homogeneously calibrated and carefully
assembled to construct an R-band light curve containing about 1340 data points
in 42 days. This extremely well-sampled optical light curve allows us to follow
the dramatic flux variability of the source in detail. In addition, we show
radio-to-UV spectral energy distributions (SEDs) at different epochs, which
represent different brightness levels. In the considered period, the source
varied by 2.6 mag in a couple of weeks in the R band. Many episodes of fast
(i.e. intranight) variability were observed, most notably on December 12, when
a flux increase of about 1.1 mag in 1.5 hours was detected, followed by a steep
decrease of about 1.2 mag in 1 hour. The contribution by the thermal component
is difficult to assess, due to the uncertainties in the Galactic, and possibly
also intrinsic, extinction in the UV band. However, polynomial fitting of
radio-to-UV SEDs reveals an increasing spectral bending going towards fainter
states, suggesting a UV excess likely due to the thermal emission from the
accretion disc.
