ObjectiveThere is a growing concern that the restrictions imposed due to the COVID-19 pandemic could lead to increased loneliness and mental disorders, which are considered a major public health problem. The aim of the study was to assess loneliness, anxiety, depression and irritability in the Polish population during the COVID-19 pandemic.DesignA cross-sectional study. The study participants completed an online questionnaire using the computer-assisted web interview technique. Data were collected using Revised UCLA Loneliness Scale and Hospital Anxiety and Depression Scale.SettingPoland.ParticipantsThis study was conducted between 6 October and 12 October 2020, in a representative sample of 890 Polish residents.Primary outcome measuresSelf-reported loneliness; sociodemographic and COVID-19 pandemic factors associated with loneliness, anxiety, depression and irritability.ResultsThe analyses showed a moderately high degree of loneliness in 22%, symptoms of anxiety in 27%, depression in 14% and irritability in 33% of the respondents. The increasing severity of anxiety, depression and irritation in the study group was accompanied by higher loneliness. Generally, younger people, both tested positive for SARS-CoV-2 and those who experienced home quarantine, scored higher in both scales.ConclusionsIt is necessary to identify those most vulnerable to loneliness, anxiety and depression during a crisis to assess health needs and proactively allocate resources during and after the pandemic. Loneliness, anxiety, depression and irritability are important factors to consider in a population of younger, disadvantaged people, who tested positive for SARS-CoV-2, people who were quarantined at home, and people who believe that their physical and mental health is worse than in the pre-pandemic period. It is important to cater for the mental health of individuals during the COVID-19 pandemic and to promote psychological interventions to improve mental well-being in potentially vulnerable social groups.