Background and Objective: Depression and anxiety represented the leading causes for disease burden among children and adolescents. COVID-19 pandemic-related restrictions and school closures have influenced the life for the European youth; however, an up-to-date review how these changes affected mental health is lacking. Therefore, we aimed to determine the change in the share of children and adolescents with depression and anxiety symptoms during COVID-19 pandemic compared to pre-pandemic baselines and evaluate country-related policy impacts. Methods: Six databases were searched using a peer-reviewed search string with citation tracking and grey literature searches. Primary outcomes were (1) general depression or anxiety symptoms and (2) clinically relevant depression or anxiety rates. The validated Oxford Stringency Index was used as indicator for pandemic-related restrictions. Screening of title/abstract and full text level was done in duplicate. Two reviewers independently assessed risk of bias and certainty of evidence, using the Grading of Recommendations Assessment, Development and Evaluation (GRADE) approach. Data were pooled using a random-effects model. PROSPERO registration: CRD42022303714. Results: Of 7,422 nonduplicate records, 22 studies for depression symptoms and 18 for anxiety symptoms with data from 1,621,166 participants pre-pandemic and 1,571,062 during pandemic, met inclusion criteria. Moderate certainty of evidence was observed for general depression symptoms (standardized mean difference [SMD], 0.21; 95% confidence interval [CI], 0.12-0.30]) and low certainty for clinically relevant depression rates (odds ratio, 1.36; 95% CI, 1.05-1.76]). For general anxiety symptoms low certainty of evidence was determined (SMD, 0.34; 95% CI, 0.17-0.51]) and very low certainty for clinically relevant anxiety rates (odds ratio, 1.08; 95% CI, 0.98-1.19). Effect estimates were considerable higher, when pandemic-related restrictions were more stringent or school closure occurred. Conclusions: Depression and anxiety symptoms increased in a pre-during-comparison in the COVID-19 pandemic in Europe, whereby pandemic-related restrictions (such as school closures) resulted in an immense effect increase.