Context : We present 8.2m VLT spectroscopic observations of 28 H ii regions in 16 emission-line galaxies and 3.6m ESO telescope spectroscopic observations of 38 H ii 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 .

Aims : 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 .

Methods : Physical conditions and element abundances in the new sample are derived with the T _ { e } -method , excluding six H ii regions from the VLT observations and nearly two third of the H ii regions from the 3.6m observations .
Element abundances for the latter galaxies were derived with the semiempirical strong-line method .

Results : From our new observations we find that the oxygen abundance in 61 out of the 66 H ii regions of our sample ranges from 12 + log O/H = 7.05 to 8.22 .
Our sample includes 27 new galaxies with 12 + log O/H < 7.6 which qualify as extremely metal-poor star-forming galaxies ( XBCDs ) .
Among them are 10 H ii regions with 12 + log O/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 H ii regions .
9000 more galaxies with oxygen abundances , based mainly on the T _ { e } -method , are compiled from the SDSS .
Photometric data for all galaxies of our combined sample are taken from the SDSS database while distances are from the NED .
Our data set spans a range of 8 mag with respect to its absolute magnitude in SDSS g ( –12 \ga M _ { g } \ga –20 ) and nearly 2 dex in its oxygen abundance ( 7.0 \la 12 + log O/H \la 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 .

Conclusions :