  [Abridged] We report on a multiwavelength observation of the blazar 3C 454.3
(which we dubbed "crazy diamond") carried out on November 2007 by means of the
astrophysical satellites AGILE, INTEGRAL, Swift, the WEBT Consortium, and the
optical-NIR telescope REM. 3C 454.3 is detected at a $\sim 19-\sigma$ level
during the 3-week observing period, with an average flux above 100 MeV of
$F_{\rm E>100MeV} = (170 \pm 13) \times 10^{-8}$ \phcmsec. The gamma-ray
spectrum can be fit with a single power-law with photon index $\Gamma_{\rm
GRID} = 1.73 \pm 0.16$ between 100 MeV and 1 GeV. We detect significant
day-by-day variability of the gamma-ray emission during our observations, and
we can exclude that the fluxes are constant at the 99.6% ($\sim 2.9 \sigma$)
level. The source was detected typically around 40 degrees off-axis, and it was
substantially off--axis in the field of view of the AGILE hard X-ray imager.
However, a 5-day long ToO observation by INTEGRAL detected 3C 454.3 at an
average flux of about $F_{\rm 20-200 keV} = 1.49 \times 10^{-3}$ \phcmsec with
an average photon index of $\Gamma_{\rm IBIS} = 1.75 \pm 0.24$ between 20--200
keV. Swift also detected 3C 454.3 with a flux in the 0.3--10 keV energy band in
the range $(1.23-1.40) \times 10^{-2}$ \phcmsec{} and a photon index in the
range $\Gamma_{\rm XRT} = 1.56-1.73$. In the optical band, both WEBT and REM
show an extremely variable behavior in the $R$ band. A correlation analysis
based on the entire data set is consistent with no time-lags between the
gamma-ray and the optical flux variations. Our simultaneous multifrequency
observations strongly indicate that the dominant emission mechanism between 30
MeV and 30 GeV is dominated by inverse Compton scattering of relativistic
electrons in the jet on the external photons from the broad line region.

Astrophysics