  The Compton Gamma-Ray Observatory (CGRO) experiments EGRET and COMPTEL
observed the Virgo sky region continuously for 7 weeks between December 10,
1996 and January 28, 1997. The prominent quasar 3C~273 was found to be the
brightest source in gamma-rays and was significantly detected by EGRET and
COMPTEL. The EGRET experiment observed a time-variable flux at energies above
100 MeV, which reached in a 2-week flaring period (December 30, 1996 to January
14, 1997) its highest flux level observed during the CGRO-era. COMPTEL,
however, does not observe obvious time variability at energies below ~30 MeV
contemporaneous to EGRET. In particular, no flare was observed, indicating that
this outburst is solely a high-energy (>100 MeV) phenomenon. The energy
spectrum between 3 MeV and 10 GeV is well represented by a simple power-law
model. Below 3 MeV a spectral turnover is indicated. Performing spectral
analysis for different time periods, we found evidence for a spectral hardening
during the flaring period, which is consistent with the flare occurring mainly
at the higher energies and with its absence at COMPTEL energies of a few MeV.
This may be interpreted as an indication that the emission in the EGRET energy
range is dominated by a different radiation mechanism than the MeV emission. We
argue that the most likely mechanism for the high-energy flare is
inverse-Compton scattering of reprocessed accretion-disk radiation.
