In everyday urban life, people see and face different problems and challenges. Citizens may face criminal activity, poor public transportation, and other difficulties. If their claims were gathered and used to highlight urban problems, they could draw attention from authorities to such issues, saving time to solve the problems and thus improving their quality of life. People would behave as (very sophisticated) sensors of the urban environment. However, allowing anyone to share and interact with others about topics of interest to them is a necessary step toward such big-picture thinking. As a proposal to reach this goal, we describe CidadeSocial, a platform for participatory and passive sensing in a dynamic urban setting. Next, we describe the main features of a developed mobile client. Afterwards, we exemplify how to use the platform in two different application scenarios. The proposed platform contributes to the development of smart and participatory communities in which everyone plays an important role in the information economy.