Generally the gas metallicity in distant galaxies can only be inferred by using a few prominent emission lines. Various theoretical models have been used to predict the relationship between emission line fluxes and metallicity, suggesting that some line ratios can be used as diagnostics of the gas metallicity in galaxies. However, accurate empirical calibrations of these emission line flux ratios from real galaxy spectra spanning a wide metallicity range are still lacking. In this paper we provide such empirical calibrations by using the combination of two sets of spectroscopic data: one consisting of low-metallicity galaxies with a measurement of Oiiiλ4363 taken from the literature, including spectra from the Sloan Digital Sky Survey (SDSS), and the other one consisting of galaxies in the SDSS database whose gas metallicity has been determined from various strong emission lines in their spectra. This combined data set constitutes the largest sample of galaxies with information on the gas metallicity available so far and spanning the widest metallicity range. By using these data we obtain accurate empirical relations between gas metallicity and several emission line diagnostics, including the R23 parameter, the Niiλ6584/Hα and Oiiiλ5007/Niiλ6584 ratios. Our empirical diagrams show that the line ratio Oiiiλ5007/Oiiλ3727 is a useful tool to break the degeneracy in the R23 parameter when no information on the Niiλ6584 line is available. The line ratio Neiiiλ3869/Oiiλ3727 also results to be a useful metallicity indicator for high-z galaxies, especially when the R23 parameter or other diagnostics involving Oiiiλ5007 or Niiλ6584 are not available. Additional, useful diagnostics newly proposed in this paper are the line ratios of (Hα+Nii$\lambda\lambda$6548,6584)/Siiλ6720, Oiiiλ5007/Hβ, and Oiiλ3727/Hβ. Finally, we compare these empirical relations with photoionization models. We find that the empirical R23-metallicity sequence is strongly discrepant with respect to the trend expected by models with constant ionization parameter. Such a discrepancy is also found for other line ratios. These discrepancies provide evidence for a strong metallicity dependence of the average ionization parameter in galaxies. In particular, we find that the average ionization parameter in galaxies increases by ~0.7 dex as the metallicity decreases from 2 $Z_{\odot}$ to 0.05 $Z_{\odot}$, with a small dispersion. This result should warn about the use of theoretical models with constant ionization parameter to infer metallicities from observed line ratios.