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The quasar-type blazar 3C 454.3 underwent a phase of high activity in summerand autumn 2007, which was intensively monitored in the radio-to-optical bandsby the Whole Earth Blazar Telescope (WEBT). The gamma-ray satellite AGILEdetected this source first in late July, and then in November-December 2007. Inthis letter we present the multifrequency data collected by the WEBT andcollaborators during the second AGILE observing period, complemented by a fewcontemporaneous data from UVOT onboard the Swift satellite. The aim is to tracein detail the behaviour of the synchrotron emission from the blazar jet, and toinvestigate the contribution from the thermal emission component. Optical datafrom about twenty telescopes have been homogeneously calibrated and carefullyassembled to construct an R-band light curve containing about 1340 data pointsin 42 days. This extremely well-sampled optical light curve allows us to followthe dramatic flux variability of the source in detail. In addition, we showradio-to-UV spectral energy distributions (SEDs) at different epochs, whichrepresent different brightness levels. In the considered period, the sourcevaried 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, whena flux increase of about 1.1 mag in 1.5 hours was detected, followed by a steepdecrease of about 1.2 mag in 1 hour. The contribution by the thermal componentis difficult to assess, due to the uncertainties in the Galactic, and possiblyalso intrinsic, extinction in the UV band. However, polynomial fitting ofradio-to-UV SEDs reveals an increasing spectral bending going towards fainterstates, suggesting a UV excess likely due to the thermal emission from theaccretion disc.

The high activity of 3C 454.3 in autumn 2007. Monitoring by the WEBT during the AGILE detection