  (abridged) We present 8.2m VLT spectroscopic observations of 28 HII regions
in 16 emission-line galaxies and 3.6m ESO telescope spectroscopic observations
of 38 HII regions in 28 emission-line galaxies. These emission-line galaxies
were selected mainly from the Data Release 6 (DR6) of the Sloan Digital Sky
Survey (SDSS) as metal-deficient galaxy candidates. We collect photometric and
high-quality spectroscopic data for a large uniform sample of star forming
galaxies including new observations. Our aim is to study the
luminosity-metallicity (L-Z) relation for nearby galaxies, especially at its
low-metallicity end and compare it with that for higher-redshift galaxies. From
our new observations we find that the oxygen abundance in 61 out of the 66 HII
regions of our sample ranges from 12+logO/H=7.05 to 8.22. Our sample includes
27 new galaxies with 12+logO/H<7.6 which qualify as extremely metal-poor
star-forming galaxies (XBCDs). Among them are 10 HII regions with
12+logO/H<7.3. The new sample is combined with a further 93 low-metallicity
galaxies with accurate oxygen abundance determinations from our previous
studies, yielding in total a high-quality spectroscopic data set of 154 HII
regions. 9000 more galaxies with oxygen abundances, based mainly on the
Te-method, are compiled from the SDSS. Our data set spans a range of 8 mag with
respect to its absolute magnitude in SDSS g (-12>Mg>-20) and nearly 2 dex in
its oxygen abundance (7.0<12+logO/H<8.8), allowing us to probe the L-Z relation
in the nearby universe down to the lowest currently studied metallicity level.
The L-Z relation established on the basis of the present sample is consistent
with previous ones obtained for emission-line galaxies.
