Context. We report the detection by the AGILE satellite of an intense gamma-ray flare from the gamma-ray source 3EG J1255-0549, associated with the Flat Spectrum Radio Quasar 3C 279, during the AGILE pointings towards the Virgo Region on 2007 July 9-13. Aims. The simultaneous optical, X-ray and gamma-ray covering allows us to study the spectral energy distribution (SED) and the theoretical models relative to the mid-July flaring episode. Methods. AGILE observed the source during its Science Performance Verification Phase with its two co-aligned imagers: the Gamma-Ray Imaging Detector (GRID) and the hard X-ray imager (Super-AGILE) sensitive in the 30 MeV-50 GeV and 18-60 keV respectively. During the AGILE observation the source was monitored simultaneously in the optical band by the REM telescope and in the X-ray band by the Swift satellite through 4 target of opportunity observations. Results. During 2007 July 9-13, AGILE-GRID detected gamma-ray emission from 3C 279, with the source at similar to 2 degrees. from the center of the field of view, with an average flux of (210 +/- 38) x 10(-8) ph cm(-2) s(-1) for energy above 100 MeV. No emission was detected by Super-AGILE, with a 3-sigma upper limit of 10 mCrab. During the observation, which lasted about 4 days, no significative gamma-ray flux variation was observed. Conclusions. The Spectral Energy Distribution is modelled with a homogeneous one-zone Synchrotron Self Compton emission plus the contributions by external Compton scattering of the direct disk radiation and, to a lesser extent, by external Compton scattering of photons from the Broad Line Region.
