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The COVID-19 pandemic has shaken the world since the beginning of 2020. Spain is among the European countries with the highest incidence of the disease during the first pandemic wave. We established a multidisciplinar consortium to monitor and study the evolution of the epidemic, with the aim of contributing to decision making and stopping rapid spreading across the country. We present the results for 2170 sequences from the first wave of the SARS-Cov-2 epidemic in Spain and representing 12% of diagnosed cases until 14th March. This effort allows us to document at least 500 initial introductions, between early February-March from multiple international sources. Importantly, we document the early raise of two dominant genetic variants in Spain (Spanish Epidemic Clades), named SEC7 and SEC8, likely amplified by superspreading events. In sharp contrast to other non-Asian countries those two variants were closely related to the initial variants of SARS-CoV-2 described in Asia and represented 40% of the genome sequences analyzed. The two dominant SECs were widely spread across the country compared to other genetic variants with SEC8 reaching a 60% prevalence just before the lockdown. Employing Bayesian phylodynamic analysis, we inferred a reduction in the effective reproductive number of these two SECs from around 2.5 to below 0.5 after the implementation of strict public-health interventions in mid March. The effects of lockdown on the genetic variants of the virus are reflected in the general replacement of preexisting SECs by a new variant at the beginning of the summer season. Our results reveal a significant difference in the genetic makeup of the epidemic in Spain and support the effectiveness of lockdown measures in controlling virus spread even for the most successful genetic variants. Finally, earlier control of SEC7 and particularly SEC8 might have reduced the incidence and impact of COVID-19 in our country., This work was funded by the Instituto de Salud Carlos III project COV20/00140, Spanish 593 National Research Council project CSIC-COV19-021 and ERC StG 638553 to IC, and BFU2017- 594 89594R to FGC. MC is supported by Ramón y Cajal program from Ministerio de Ciencia and 595 grants RTI2018-094399-A-I00 and SEJI/2019/011. 596 We gratefully acknowledge Hospital Universitari Vall d'Hebron, Instituto de Salud Carlos 597 III, IrsiCaixa AIDS Research Lab and all the international researchers and institutions that 598 submitted sequenced SARS-CoV-2 genomes to the GISAID’s EpiCov™ Database, as an 599 important part of our analyses have been made possible by the share of their work.