Citizen science offers a huge potential to complement official responses to the Coronavirus pandemic, both in terms of facilitating scientific advances and of improving public engagement. Here we provide an analysis of Covid-19 citizen science projects using a two-phase research approach. Website content analysis was used to understand the outstanding characteristics of citizen science projects that endeavoured to respond to the pandemic, in terms of their geographical distribution, aims, design and characteristics of citizen engagement. Followed by detailed case studies of seven citizen science initiatives which provided in-depth understanding of the development of projects, and practices used across a range of approaches. The website review revealed that the covid-19 citizen science landscape was dominated by contributory crowd-sourcing projects, mainly related to health sciences. Most of the projects were purpose-developed by scientists to respond to the global Covid-19 crisis, and leveraged basic information technologies (internet, smartphones) to collect self-reported data on people’s symptoms, behaviour or well-being from questionnaires. More than half of all the projects were global in scope, but projects were found at all geographical scales. Often, the effort required by participants was non-negligible, involving repeated inputs at frequent intervals, though of limited duration. The review also suggested data accessibility and validation procedures were limited. The case studies revealed the importance of preparedness to respond to new challenges, building on existing experience, collaborations and modular software infrastructure. It was interesting to notice that simple designs and partnerships with NGOs seemed to improve the likelihood that projects would reach high levels of engagement. Additionally, all case studies highlight the importance of regular and honest feedback, to secure the trust and engagement of the participants. Recommendations from several projects include not being afraid to plan for success, thinking big, open and collaboratively when designing a project.