Data collected during the one year project (2018-2019) Citizen Science in Action promoted by the Barcelona Network of Public Libraries (Spain). The project was structured around two interdependent and hands-on activities offered to librarians as part of their lifelong training. The activities were: the Citizen Science Lab; and Science and Citizen Action. The Citizen Science Lab consisted of an introductory course of 5 two-hour citizen science capacitation sessions addressed at 30 librarians from 24 different libraries. The activity Science and Citizen Action involved 7 library professionals from three different libraries and their community of users and local associations in the co-design and implementation of a public behavioral experiment. The data covers the relatioship between librarians and library users' attitudes towards citizen science, their motivations to participate in a citizen science project, their perceived ability to implement a citizen science project (as for librarians) or to contribute to science (as for library users), and the participants intention to keep engaged with citizen science. For the Citizen Science Lab data on librarians engagement was collected through paper-based questionnaires at the beginning and at the end of the activity. For the Science and Citizen Action data on librarians engagement was collected at the beginning, middle and end of the activity. The Project was approved by the Ethics Committee of Universitat de Barcelona, and all methods were performed in accordance with the relevant guidelines and regulations.