  The giant radio galaxy M87 is usually classified as a Fanaroff-Riley class I
source, suggesting that M87 is a mis-aligned BL Lac object. Its unresolved
nuclear region emits strong non-thermal emission from radio to X-rays which has
been interpreted as synchrotron radiation. In an earlier paper we predicted M87
as a source of detectable gamma ray emission in the context of the hadronic
Synchrotron-Proton Blazar (SPB) model. The subsequent tentative detection of
TeV energy photons by the HEGRA-telescope array would, if confirmed, make it
the first radio galaxy to be detected at TeV-energies. We discuss the emission
from the unresolved nuclear region of M87 in the context of the SPB model, and
give examples of possible model representations of its non-simultaneous
spectral energy distribution. The low-energy component can be explained as
synchrotron radiation by a primary relativistic electron population that is
injected together with energetic protons into a highly magnetized emission
region. We find that the gamma-ray power output is dominated either by
mu/pi-synchrotron or proton synchrotron radiation depending on whether the
primary electron synchrotron component peaks at low or high energies,
respectively. The predicted gamma-ray luminosity peaks at around 100 GeV at a
level comparable to that of the low-energy hump, and this makes M87 a promising
candidate source for the newly-commissioned high-sensitivity low-threshold
Cherenkov telescopes H.E.S.S., VERITAS, MAGIC and CANGAROO III. (abridged)
