An atomistic model of SiO2/BaTiO3 interface was constructed using ab initio molecular dynamics. Analysis of its atomistic structure and electronic properties reveals that (i) the band gap at the stoichiometric SiO2/BaTiO3 interface is significantly smaller than that of the bulk BaTiO3 and SiO2 and (ii) the interface contains ~5.5 nm-2 oxygen vacancies (V2+) in the outermost TiO2 plane of the BaTiO3 and ~11 nm-2 Si-O-Ti bonds resulting from breaking Si-O-Si and Ti-O-Ti bonds and subsequent rearrangement of the atoms. This structure gives rise to the interface polar region with positive and negative charges localized in the BaTiO3 and SiO2 parts of the interface, respectively. We propose that high dielectric response, observed experimentally in the SiO2-coated nanoparticles of BaTiO3, is due to the electron gas formed in oxygen-deficient BaTiO3 and localized in the vicinity of the polar interface.