Context : We report the detection by the AGILE satellite of an intense gamma-ray flare from the gamma-ray source 3EG J1255-0549 , associated to 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 flaring episode of mid-July .

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 optical band by the REM telescope and in the X-ray band by the Swift satellite through 4 ToO observations .

Results : During 2007 July 9-13 July 2007 , AGILE-GRID detected gamma-ray emission from 3C 279 , with the source at \sim 2 ^ { \circ } from the center of the Field of View , with an average flux of ( 210 \pm 38 ) \times 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 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 direct disk radiation and , to a lesser extent , by external Compton scattering of photons from the Broad Line Region .