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We present the results of the analysis of an extensive set of new and literature high quality data concerning Fe, C, N, O, Na, and Mg. This analysis exploited the T_eff scale determined in Gratton et al. (1996A&A...314..191G), and the non-LTE abundance corrections computed in Gratton et al. (1999A&A...350..955G).
Result. obtained with various abundance indices are discussed and compared. Detailed comparison with models of galactic chemical evolution will be presented in future papers of this series. Our non-LTE analysis yields the same O abundances from both permitted and forbidden lines for stars with T_eff>4600K, in agreement with King (1993AJ....106.1206K), but not with other studies using a lower T_eff -scale for subdwarfs. However, we obtain slightly smaller O abundances for the most luminous metal-poor field stars than for fainter stars of similar metallicities, an effect attributed to inadequacies of the adopted model atmospheres (Kurucz 1992 (priv. comm.), with overshooting) for cool stars. We find a nearly constant O overundance in metal-poor stars ([Fe/H]←0.8), at a mean value of 0.46±0.02dex (σ=0.12, 32 stars), with only a gentle slope with [Fe/H] (~-0.1); this result is different from the steeper slope recently obtained using OH band in the near UV. If only bonafide unmixed stars are considered, C abundances scale with Fe ones (i.e. [C/Fe]≃0) down to [Fe/H]~-2.5. Due to our adoption of a different T_eff scale, we do not confirm the slight C excess in the most metal poor disk dwarfs (-0.8<[Fe/H]←0.4) found in previous investigations. Na abundances scale as Fe ones in the high metallicity regime, while metal-poor stars present a Na underabundance. None of the field stars analyzed belong to the group of O-poor and Na-rich stars observed in globular clusters. Na is deficient with respect to Mg in halo and thick disk stars; within these populations, Na deficiency may be a slow function of [Mg/H]. Solar [Na/Mg] ratios are obtained for thin disk stars.