Although IEEE 802.15.4 is being considered as a promising standard for low-cost low-power Wireless Sensor Networks (WSNs), several issues in the specification are still open. One of those open issues is how to build a synchronized multi-hop mesh network for power efficient, scalable, and robust networking. In fact, while the current specification supports multi-hop networking using peer-to-peer topology, it restricts its use to the beaconless mode. This paper presents an extension to the current IEEE 802.15.4 standard in which mesh networking in the low power beacon mode is enabled through the use of distributed beacon scheduling. The work presented in this paper has been implemented on top of the Open-ZB IEEE 802.15.4 implementation within the TinyOS operating system. The feasibility of our proposal is demonstrated and evaluated through both physical test-bed and computer based emulation.