According to most estimates, the Jewish population of Europe was approximately 9.5 million in 1939. By 1945, it is estimated that around 3.5 million European Jews had been killed during the Holocaust, leaving a remaining Jewish population of approximately 6 million in Europe. This means that around 37% of the Jewish population was killed during WWII.

the most commonly cited estimate for the pre-war Jewish population of Europe is approximately 9.5 million. This figure includes both Ashkenazi and Sephardic Jews living in Europe at the time.

By the end of the war in 1945, it is estimated that approximately 6 million Jews had been killed as a result of the Holocaust. This figure represents the vast majority of European Jews and a significant portion of the global Jewish population.

The percentage of Jews killed during the Holocaust, as a proportion of the pre-war Jewish population in Europe, was approximately 63%. However, it is important to note that this percentage varied significantly depending on the country or region in question. For example, in some areas of Poland and Lithuania, the Jewish population was almost entirely wiped out, while in other countries such as Italy and Bulgaria, relatively few Jews were killed.

Sources:
United States Holocaust Memorial Museum. (n.d.). Holocaust Encyclopedia: Jews in Europe Before the Holocaust. Retrieved from https://encyclopedia.ushmm.org/content/en/article/jews-in-europe-before-the-holocaust
United States Holocaust Memorial Museum. 

(n.d.). Holocaust Encyclopedia: Jewish Losses During the Holocaust. Retrieved from https://encyclopedia.ushmm.org/content/en/article/jewish-losses-during-the-holocaust
Holocaust Education & Archive Research Team. (2017). Holocaust Statistics. Retrieved from
http://www.holocaustresearchproject.org/othercamps/holocauststatistics.html

The World Holocaust Remembrance Center. (2018). The Holocaust. Retrieved from https://www.yadvashem.org/holocaust/about.html
BBC. (2011). Holocaust: Who are the missing million? Retrieved from
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