The possibility to manipulate the electrical properties of matter with very short optical pulses is a fascinating field of research with possible far-reaching applications in many relevant technological fields1. The first step towards the realization of this goal is to understand the ultrafast dynamics at the basis of light-matter interaction. Short and intense pulses allowed us to investigate a very interesting regime where the photon energy becomes comparable to the cycle-averaged kinetic energy of the electrons in the field. As the optical response of the material transitions from a classical to a quantum-mechanical description many intriguing effects co-exist and the importance of inter- versus intra-band transitions is debated. We used attosecond transient absorption spectroscopy in polycrystalline diamond to study its response in this regime. THe experimental results revealed close-to-petahertz electron dynamics high into the conduction band. Comparison with time-dependent density functional theory calculations and a simpler two band numerical model allowed to identifiey the dynamical Franz-Keldysh effect as the main physical mechanism underlying the observed response. Therefore our results and analysis constitutes an important step towards a full understanding of the response of dielectrics to Petahertz fields.