  We report the extraordinary gamma-ray activity (E>100 MeV) of the
gravitationally lensed blazar PKS 1830-211 (z=2.507) detected by AGILE between
October and November 2010. The source experienced on October 14 a flux increase
of a factor of ~ 12 with respect to its average value and kept brightest at
this flux level (~ 500 x 10^{-8} ph cm^-2 sec^-1) for about 4 days. The 1-month
gamma-ray light curve across the flare showed a mean flux F(E>100 MeV)= 200 x
10^{-8} ph cm^-2 sec^-1, which resulted in an enhancement by a factor of 4 with
respect to the average value. Following the gamma-ray flare, the source was
observed in NIR-Optical energy bands at the Cerro Tololo Inter-American
Observatory and in X-rays by Swift/XRT and INTEGRAL/IBIS. The main result of
these multifrequency observations is that the large variability observed in
gamma-rays has not a significant counterpart at lower frequencies: no variation
greater than a factor of ~ 1.5 resulted in NIR and X-ray energy bands. PKS
1830-211 is then a good "gamma-ray only flaring" blazar showing substantial
variability only above 10-100 MeV. We discuss the theoretical implications of
our findings.
