  (Abridged) We present fine structure and recombination lines observations of
the ionized gas toward a sample of 18 sources located within 300 pc of the
center of the Galaxy (GC). The fine structure lines from [NII] and [SIII] have
been detected in 16 sources. In 10 sources we have even detected the [OIII] 88
$\mu$m line. The measured N, S and Ne abundances are similar to those measured
in the HII regions in the 5-kpc ring and in the nuclei of starburst galaxies.
The fine structure lines ratios measured for all the sources can be explained
by photo-ionization with an effective temperature of 32000-37000 K and an
ionization parameter, $U$, of $-1>\log U > -3$. The highest excitation is found
in the Radio Arc region but there must be more ionizing sources distributed
over the Galactic center than the known clusters of massive stars. Most of the
clouds are located far from the prominent continuum complexes (Sgr C, B ...).
However, it is possible that the clouds are ionized by escaped photons from
those complexes. The excitation ratios, effective temperature and ionization
parameter measured in the GC are similar to those found in some low excitation
starburst galaxies. The [NeIII]/[NeII] line ratios measured in the GC sources
are consistent with the results of the Thornely et al. (2000) model for a short
burst of massive star formation less than 8 Myr ago. We have also found that
the [NeII] 13 $\mu$m ~ to far-infrared continuum ratio measured for the GC
sources is similar to that of external galaxies, supporting the idea by Sturm
et al. (2002) that the far-infrared continuum in Active Galaxies is dominated
by dust heated by stellar radiation rather than by the AGN.
