The last few years, the fast democratization of mobile media devices, and in particular the smartphones, came along with a reflection by urban public actors on the possibilities of renewing citizen participation with this technology. Collaborative cartographies are co-organized by cities, inventories of urban components, measurement campaigns of air pollution or noise pollution have flourished. Those local public actions, initiated by local authorities, expertise and associations, are close to citizen science methods. To analyze this emerging citizen participation approaches using mobile media devices it seems to us relevant to use the concept of citizen as sensor. This concept appeared in the scientific literature on volunteered geographic information and describes an interaction between an inhabitant, a technology and his environment: the citizen produces and shares some data on his environment to participate to common knowledge. Transposed to the urban studies, we show that the concept of citizen as sensor combines social interactions, human-computer interactions and human-environment interactions. The observations of those interactions are made in some specific situations during the citizen participation workshops: urban walks are usually organized to make measurement, inventories in group. From the analysis of these interactions, we will present how hybrid workshops (digital and physical) put in debate perceptions and representations of urban environment, by creating space and time for debate and discussion on observations, descriptions and characterizations of urban space. These results arise from participant observations on three case studies in the west of France.