Objectives: Because of the COVID-19 pandemic, societies have been shut down, changing the lives of citizens worldwide, including their physical activity (PA) behavior. This cross-sectional study aimed to investigate the impact of the shutdown on Danish citizens' leisure PA throughout different stages of life: 15–18, 19–29, 30–59, and 60+ years.Methods: Between April 3 and 15, 2020, while Denmark was shut down, a survey was distributed through online platforms. Danish citizens (>15 years) could participate in the study, answering questions about their PA behavior before, and during the shutdown.Results: The number of total participants was 1,802; 7.9% teens, 21.5% younger adults, 58.7% adults, and 11.9% older adults. Mean minutes of PA decreased 16.1% from before to during the shutdown. Teens had the largest decrease in PA (36.6%) followed by older (24.9%) and younger adults (21.3%). Low educated (31.5%) and those living in rural areas (30.9%) experienced the largest decrease in PA. Main factors for not doing PA during the shutdown were that they missed what they used to do, lacked social support, and did not have access to the right facilities.Conclusions: During the shutdown, the Danish population struggled even more to comply with national PA guidelines compared with before the shutdown. Although social distancing and shutting down sports facilities are important for preventing the spread of the virus, it can have negative consequences for teens' and younger and older adults' opportunities and motivation for PA, leading to an alarming decrease in PA, and, consequently, will have major public health consequences.