BACKGROUND: SARS-CoV-2 has rapidly spread worldwide, threatening public health and financial and social life. AIMS: The current study's aim was to determine the prevalence of psychological distress and post-traumatic stress symptoms in the Greek population during the first COVID-19 lockdown, and to detect potential correlates. METHOD: An anonymous online survey was conducted between 10 April and 4 May 2020, to collect information regarding people's psychological functioning and COVID-19-related perceptions. RESULTS: A total of 1443 individuals completed the survey; 293 (20%) reported clinically significant anxiety symptoms, 188 (12.9%) reported clinically significant depressive symptoms and 506 (36.4%) suffered from definite post-traumatic stress disorder. Anxiety symptoms were independently associated with female gender (ß = 1.281, 95% CI 0.808-1.755, P < 0.001), educational level (ß = -1.570, 95% CI -2.546 to -0.595, P = 0.002), perceived severity (ß = -1.745, 95% CI -3.146 to -0.344, P = 0.015) and COVID-19-related worry (ß = 7.633, 95% CI 6.206-9.060, P < 0.001). Depressive symptoms were strongly correlated with educational level (ß = -1.298, 95% CI -2.220 to -0.377, P = 0.006), perceived severity (ß = -1.331, 95% CI -2.579 to -0.082, P = 0.037) and COVID-19-related worry (ß = 4.102, 95% CI 2.769-5.436, P < 0.001). Finally, post-traumatic stress symptoms were linked to female gender (ß = 6.451, 95% CI 4.602-8.299, P < 0.001), educational level (ß = -5.737, 95% CI -9.479 to -1.996, P = 0.003), psychiatric history (ß = -4.028, 95% CI -6.274 to -1.782, P < 0.001) and COVID-19-related worry (ß = 23.865, 95% CI 18.201-29.530, P < 0.001). CONCLUSIONS: A significant percentage of the population reported clinically important anxiety, depressive and post-traumatic stress symptoms. Women, less-educated individuals and people with a psychiatric history appeared more vulnerable to the pandemic's psychological impact.