Mobile Sensor Networks based on connected vehicles and smart phones are poised to become key enablers in the development of sustainable and intelligent transportation systems in urban environments. By gathering and processing massive amounts of data in real-time, this form of information and communication infrastructure can be instrumental towards improving traffic flow, reducing carbon emissions and promoting multi-modal mobility and enhanced coordination among public transit systems. We propose a system architecture for a Massive Multi-Sensor Urban Scanner capable of acquiring large quantities of real-time information from a vast variety of sources and sending the data to a back-end data processing cloud using multiple communication interfaces. Requirements, technical challenges, design choices and first results are explained in detail based on a prototype that is currently being deployed in Porto, Portugal.