  We analyze the conditions that enable acceleration of particles to ultra-high
energies, ~10^{20} eV (UHECRs). We show that broad band photon data recently
provided by WMAP, ISOCAM, Swift and Fermi satellites, yield constraints on the
ability of active galactic nuclei (AGN) to produce UHECRs. The high energy (MeV
- GeV) photons are produced by Compton scattering of the emitted low energy
photons and the cosmic microwave background or extra-galactic background light.
The ratio of the luminosities at high and low photon energies can therefore be
used as a probe of the physical conditions in the acceleration site. We find
that existing data excludes core regions of nearby radio-loud AGN as possible
acceleration sites of UHECR protons. However, we show that giant radio lobes
are not excluded. We apply our method to Cen A, and show that acceleration of
protons to ~10^{20} eV can only occur at distances >~ 100 kpc from the core.
