The coordinated terrorist attacks on October 7, 2023, resulted in catastrophic atrocities, and marked the beginning of the 2023 Israel–Hamas war. The overwhelming coverage by mainstream and social media, characterized by extreme details and graphic images, vividly transported viewers to the horrifying scene. It remains unclear to what extent such indirect exposure influences the occurrence of stress, anxiety, and post-traumatic symptoms. We analyzed data from a three-year prospective study in which 4,797 participants received smartwatches and completed daily questionnaires, supplemented by a nationwide clinical survey with 2,536 participants. Among the participants not directly exposed, we estimated PTSD prevalence to be 22.9-36.0% and moderate to severe anxiety prevalence to be 22.9-55.32%, with 752,057 daily questionnaires before and after October 7 further indicating higher stress levels than those reported in previous events, including political disputes, the COVID-19 pandemic, and past armed conflicts. The occurrences of PTSD and anxiety are well explained by increased and persistent news consumption, and especially by the availability of gory videos on social media. Continuous monitoring of participants via smartwatches and daily questionnaires further revealed considerable differences in stress, mood, step counts, sleep quality, and duration in the first week after the October 7 events among those who later developed PTSD. This study demonstrates the unprecedented amplifying effect of mass media on mental health in terror and war settings and highlights the potential of continuous monitoring for early detection and prompt treatment of those in need.