A prominent outburst of the flat spectrum radio quasar 3C 454.3 was observed in 2014 June with the Fermi Large Area Telescope. This outburst was characterized by a three-stage light-curve pattern---plateau, flare and post-flare---that occurred from 2014 May to July, in a similar pattern as observed during the exceptional outburst in 2010 November. The highest flux of the outburst reported in this paper occurred during 2014 June 7--29, showing a multiple-peak structure in the light-curves. The

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... erage flux in these 22 days was found to be F[E > 100 MeV] = (7.2 ± 0.2) × 10^-6 ph cm^-2 s^-1, with a spectral index, for a simple power law, of Γ = 2.04 ± 0.01. That made this outburst the first γ-ray high state of 3C 454.3 ever to be detected by Fermi with such a hard spectrum over several days. The highest flux was recorded on 2014 June 15, in a 3 hr bin, at MJD 56823.5625, at a level of F[E > 100 MeV] = (17.6 ± 1.9) × 10^-6 ph cm^-2 s^-1. The rise time of one of the short subflares was found to be T_r= 1200 ± 700 s at MJD = 56827, when the flux increased from 4 to 12 × 10^-6 ph cm^-2 s^-1. Several photons above 20 GeV were collected during this outburst, including one at 45 GeV on MJD 56827, constraining the γ-ray emission region to be located close to the outer boundary of the broad-line region, leading to fast flux variability.