One of the most promising growth and deployment domains of Machine-to-Machine (M2M) communications is within the context of Smart Grid and its Advanced Metering Infrastructure. Data generation, collection, monitoring, and processing potentials are expected to be greatly expanded. An accompanying security for the communications infrastructure is generally agreed to be unmet by the legacy power grid networking protocols. In this paper, we present a new customized, secure, and scalable M2M data collection protocol for the Smart Grid. We use a hierarchical architecture with intermediary nodes collecting and relaying the data securely from measurement devices back to the power operator. While the data collectors verify the integrity, they are not given access to the content, which may also pave the way for third party providers to deliver value-added services or even the data collection itself. We also report some preliminary experimental results from a testbed developed from Raspberry Pi devices to emulate the resource-constrained measurement devices.