  We study numerically particle acceleration by the electric field induced near
the horizon of a rotating supermassive (M \sim 10^9-10^{10} M_{\odot}) black
hole embedded in the magnetic field B. We find that acceleration of protons to
energy E > 10^{20} eV is possible only at extreme values of M and B. We also
find that the acceleration is very inefficient and is accompanied by a broad
band MeV-TeV radiation whose total power exceeds at least by a factor of 1000
the total power emitted in ultra-high energy cosmic rays (UHECR). This implies
that if O(10) nearby quasar remnants were sources of proton events with energy
E > 10^{20} eV, then each quasar remnant would overshine e.g. the Crab nebula
by more than two orders of magnitude in the TeV energy band. Recent TeV
observations exclude this possibility. A model in which O(100) sources are
situated at 100-1000 Mpc is not ruled out and can be experimentally tested by
present TeV gamma-ray telescopes. Such a model can explain the observed UHECR
flux at moderate energies E \sim (4-5) 10^{19} eV.
