  We study the properties of the emission line regions in two samples of low
luminosity radio-galaxies (LLRG), focusing on the compact emission line region
(CELR) revealed to be a characteristic feature of these objects by HST
narrow-band imaging. We find a strong correlation between line and optical
continuum nuclear emission, suggesting that the optical cores (most likely of
non thermal origin) can be directly associated to the source of ionizing
photons, i.e. that we are seeing a jet-ionized narrow line region. A photon
budget argument indicates that the optical nuclear sources produce a sufficient
photon flux provided that the covering factor of the circum-nuclear gas is
rather large, on average ~ 0.3. Analysis of HST images and spectra suggests
that the CELR may take the form of a pc-scale, high filling factor, structure,
possibly an optically thin torus. Estimates of the CELR mass lead to values as
small as 10 - 1000 solar masses and photon counting sets a limit to the BLR
mass of 0.01 solar masses. When considered together with the low accretion rate
and the tenuous torus structure, a general paucity of gas in the innermost
regions of LLRG emerges as the main characterizing difference from more
powerful AGN.
