Anti-police (#EndSARS) protests took place in October 2020 across several Nigerian cities, resulting in deaths, injuries and loss of property, but the psychological sequelae of these protests have not been studied. In a cross-sectional online survey, we collected data on psychiatric morbidity and potential risk factors from 426 Nigerian social media users, who self-identified as participants or non-participants in a recent anti-police protest. We found elevated rates of psychiatric morbidity, with worse outcomes for protesters compared to non-protesters (psychological distress [44.2% vs 29.8%], depression [26.0% vs 14.9%], anxiety [51.0% vs 29.8%], post-traumatic stress disorder [7.4% vs 1.8%], current substance use [11.2% vs 4.5%] and suicidal ideation [7.1% vs 4.4%], respectively). After adjusting for the effects of COVID-19 pandemic and previous mental health diagnosis, the predictors of poor mental health among non-protesters were being unmarried (odds ratio = 7.4, p = 0.01) and low resilience (odds ratio range = 4.1-5.4, p < 0.03) while for the protesters, the predictors were low resilience (odds ratio range = 2.9-4.7, p < 0.01), being from Northern Nigeria (odds ratio = 4.7, p < 0.01) or residing in Northern Nigeria (odds ratio = 2.8, p = 0.03), being under-/unemployed (odds ratio range = 2.1-2.5, p < 0.04), holding the view that the protest was caused by the state of the economy (odds ratio = 2.0, p = 0.01), belief that the protest had a direct negative impact on the protester (odds ratio = 2.3, p = 0.04) and willingness to participate in future protests (odds ratio = 4.13, p = 0.02). We conclude that participating in the #EndSARS protest was associated with significant psychiatric morbidity. There is need to recognise and address the mental health sequelae of collective actions, and invest in programmes that build resilience and address socio-political determinants of mental health, especially with a focus on youth.