This report documents the collaborative research concerning recent urban transformations related to mass tourism in Lisbon (Portugal). The research was conducted under COESO (Collaborative Engagement on Societal Issues), a citizen science project which involved small scale pilot projects with high intensity collaboration. The pilot project "Mass tourism's impact on urban communities" resulted from the collaboration of researchers from CRIA (Centre for Research in Anthropology), and members of the environmentalist NGO ZERO (Association for the Sustainability of the Earth System). An anthropologist from the CNRS (Centre National de la Recherche Scientifique / French National Centre for Scientific Research), also working within the COESO project, observed and documented the collaborative processes of our citizen science pilot project, and contributed to the fieldwork. Through this pilot, CRIA and ZERO were involved in a co-creation process in which researchers, experts and activists jointly discuss and define objectives, scope and methods of research. Universidade Autónoma de Lisboa (UAL) and the Junta de Freguesia de Santo António (Santo Antonio Parish Council, one of Lisbon’s 24 civil parishes) were local partners, providing logistical support and being involved in part of the activities (see section 2 of this report). (Please note: Links on the document cannot be opened through the Zenodo preview. Please download the document to access the links.)