The lockdown period (March-May 2020) during the COVID-19 pandemics in Europe led to a reduction in the anthropogenic emissions of primary pollutants. For ¾ of over 1100 available monitoring stations, the average NO2 concentrations decreased by at least 2.7 µg.m-3 (or 25%) compared to the average concentrations recorded during the same period of the previous seven years. The O3 response differed spatially, with positive anomalies in northern Europe and negative anomalies in southwestern Europe. Reduced cloudiness and related enhanced radiation in northern Europe played a significant role in the increase of surface ozone concentrations by shifting the photochemical partitioning between NO2 and O3 toward more ozone. The level of total oxidant (Ox = O3 + NO2) remained unchanged except in southwestern Europe where it decreased. Several episodes lasting a few days of high level of total oxidants were observed in northern Europe. Our results illustrate the complexity of the atmospheric response to the unprecedented reduction in the emission of primary pollutants.