With the proliferation of web services in business and as the number of web services is increasing, it is anticipated that a single web service will become insufficient to handle multitude, heterogeneous, and complex functions. Hence, web service composition will be used to create new value added services with a wide range of functionalities. Management of a composed web service is a complex issue compared to the management of a non-composed (basic) web service. In this paper, we propose a multi-observer architecture for detecting and locating faults in composed web services. It makes use of a network of observers that cooperate together to observe a composed web service. An observation strategy based on a set of heuristics is presented to reduce the number of web services to be observed. Observers are developed as mobile agent observers to help reducing the load introduced by the observation. Algorithms for fault detection, notification, and collaboration between observers are described. Finally, the architecture is illustrated through a case study for observing a composed teleconferencing web services in a 3G network. Different components of the architecture are developed. The network load introduced by the observation is measured and the fault detection capabilities of the architecture are discussed.