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The structure, kinematics, and chemical composition of the far side of theMilky Way disk, beyond the bulge, are still to be revealed. Classical Cepheids(CCs) are young and luminous standard candles. We aim to use awell-characterized sample of these variable stars to study the present timeproperties of the far side of the Galactic disk. A sample of 45 Cepheidvariable star candidates were selected from near infrared time seriesphotometry obtained by the VVV survey. We characterized this sample using highquality near infrared spectra obtained with VLT/X-Shooter, deriving radialvelocities and iron abundances for all the sample Cepheids. This allowed us toseparate the CCs, which are metal rich and with kinematics consistent with thedisk rotation, from type II Cepheids (T2Cs), which are more metal poor and withdifferent kinematics. We estimated individual distances and extinctions usingVVV photometry and period-luminosity relations, reporting the characterizationof 30 CCs located on the far side of the Galactic disk, plus 8 T2Cs mainlylocated in the bulge region, of which 10 CCs and 4 T2Cs are new discoveries.This is the first sizeable sample of CCs in this distant region of our Galaxythat has been spectroscopically confirmed. We use their positions, kinematics,and metallicities to confirm that the general properties of the far disk aresimilar to those of the well-studied disk on the solar side of the Galaxy. Inaddition, we derive for the first time the radial metallicity gradient on thedisk's far side. Considering all the CCs with $R_{\mathrm{GC}} < 17\,\rm{kpc}$,we measure a gradient with a slope of $-0.062 \, \mathrm{dex\, kpc^{-1}}$ andan intercept of $+0.59 \, \rm{dex}$, which is in agreement with previousdeterminations based on CCs on the near side of the disk.

Using classical Cepheids to study the far side of the Milky Way disk: I. Spectroscopic classification and the metallicity gradient