The structural phase transition of SrTiO3 at 105 K, which has been believed to be independent of the ferroelectric soft mode [Phys. Rev. 177, 858 (1969)PHRVAO0031-899X10.1103/PhysRev.177.858], is shown, on the contrary, to be driven by the same long-wavelength polar instability. Isotope replacement of O16 by O18 is predicted to cause an increase in the structural phase transition temperature by 3.8 K. In both isotopic cases, dynamical polarizability-induced ferroelastic-type cluster formation takes place above the structural phase transition, which is intrinsic and a consequence of electron-lattice driven mode-mode coupling. Distinct length and time scales are identified. The precursor domains are evidence that order-disorder effects coexist with displacive dynamics.