Context : We report the detection by the AGILE ( Astro-rivelatore Gamma a Immagini LEggero ) satellite of an intense gamma-ray flare from the source AGL J1511-0909 , associated with the powerful quasar PKS 1510-089 , during ten days of observations from 23 August to 1 September 2007 .

Aims : During the observation period , the source was in optical decrease following a flaring event monitored by the GLAST-AGILE Support Program ( GASP ) of the Whole Earth Blazar Telescope ( WEBT ) .
The simultaneous gamma-ray , optical , and radio coverage allows us to study the spectral energy distribution and the theoretical models based on the synchrotron and inverse Compton ( IC ) emission mechanisms .

Methods : AGILE observed the source with its two co-aligned imagers , the Gamma-Ray Imaging Detector and the hard X-ray imager Super-AGILE sensitive in the 30 MeV \div 50 GeV and 18 \div 60 keV bands , respectively .

Results : Between 23 and 27 August 2007 , AGILE detected gamma-ray emission from PKS 1510-089 when this source was located \sim 50 ^ { \circ } off-axis , with an average flux of ( 270 \pm 65 ) \times 10 ^ { -8 } photons cm ^ { -2 } s ^ { -1 } for photon energy above 100 MeV .
In the following period , 28 August - 1 September , after a satellite re-pointing , AGILE detected the source at \sim 35 ^ { \circ } off-axis , with an average flux ( E > 100 MeV ) of ( 195 \pm 30 ) \times 10 ^ { -8 } photons cm ^ { -2 } s ^ { -1 } .
No emission was detected by Super-AGILE , with a 3- \sigma upper limit of 45 mCrab in 200 ksec .

Conclusions : The spectral energy distribution is modelled with a homogeneous one-zone synchrotron self Compton ( SSC ) emission plus contributions by external photons : the SSC emission contributes primarily to the X-ray band , whereas the contribution of the IC from the external disc and the broad line region match the hard gamma-ray spectrum observed .