Background: The SARS-CoV-2 pandemic first reached Brazil in late February 2020. Since then, it is one of the most affected countries globally. The rise of mental health problems in the population directly or indirectly affected is now a major concern. In the present study, we show the results of the first wave of the COVIDPsiq cohort study, which started early on (April 2020). The aim of this study was to investigate and compare independent predictors of symptoms of stress, anxiety, depression, and post-traumatic stress disorder (PTSD) in Brazilians one month after social distancing measures were implemented. Methods: Depression, Anxiety, and Stress Scale (DASS-21) and Posttraumatic Stress Disorder Checklist for DSM-5 (PCL-5) were the two self-report instruments used to assess the symptoms. Variables related to demographics, social distancing, exposure to the news of coronavirus pandemic, substance use, and traumatic situations were also analyzed. Data was gathered through a web-based survey. Multivariate analyses were performed through hierarchical multiple linear regression models (HMLR). Result: A sample of 3,587 participants completed the questionnaire, among which approximately two-thirds considered that their mental health worsened after the beginning of the social restriction measures. Stress, anxiety, depression, and PTSD levels were highly correlated with this self-perception. The five most important predictors of psychological distress were the distress related to pandemic news, younger age, current psychiatric diagnosis, trouble sleeping, and emotional abuse or violence. Limitations: This is an internet survey-based study in which convenience sample bias may limit its external validity. It does not represent the northern regions of the country and most participants are white wealthier females. It was not possible to evaluate if symptoms are clinically relevant to infer an increased incidence of mental illness. Conclusions: These results confirm the hypothesis that a pandemic would have important impacts on the mental health of the population and indicate the level of distress related to the media as an important predictor of mental symptoms.