  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. 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. 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 - 50 GeV and 18 - 60 keV
bands, respectively. Between 23 and 27 August 2007, AGILE detected gamma-ray
emission from PKS 1510-089 when this source was located about 50 degrees
off-axis, with an average flux of (270 +/- 65) x 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 about 35
degrees off-axis, with an average flux (E > 100 MeV) of (195 +/- 30) x 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. 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.
