This paper presents an emergency communication system that is able to quickly deliver emergency messages over an unreliable and best-effort network such as the Internet. The proposed architecture employs application-layer multicast to rapidly deliver emergency traffic without the support of a dedicated network infrastructure. We introduce a distributed overlay tree construction and maintenance mechanism that produces consistent, loop-free and self-adaptive trees that dynamically change over time to effectively deal with varying network conditions and offer low message delays to end nodes. We evaluate the performance of the proposed approach through an experimental study conducted on a real-life networking testbed. © 2010 IEEE.