Relativistic hadronic plasmas have an intriguing property, coined as hadronic supercriticality,
according to which they can abruptly and efficiently release the energy stored in protons through
photon outbursts. These photon flares may have a direct analogy to those observed from compact
astrophysical objects, such as Gamma Ray Bursts (GRBs). Here, we investigate for the first time
the manifestation and properties of hadronic supercriticality in adiabatically expanding sources.
We consider the injection of relativistic protons in an expanding spherical volume with a radially
decaying magnetic field and seek the parameters (e.g., proton injection luminosity) that drive the
system to supercriticality. We then perform a Monte Carlo simulation for selecting the parameters
of expanding, supercritical blobs and calculate the time-dependent electromagnetic signal from
each one of them. We compute the γ-ray light curves and broadband photon spectra from the
superposition of these blobs and present a physical picture for the prompt emission of a typical
long GRB. We also provide the all-flavour neutrino fluxes expected in this model and compare
them with those from the standard internal-shock scenario for GRBs.