Born in Kraków in Poland, Seidel was educated in a heder and yeshiva, before attending the College of Economic Studies in Vilnius. A member of the Akiva youth movement, he was involved with the anti-Nazi underground movement in the Vilna Ghetto, and headed the underground in the Klooga concentration camp. After World War II he headed the Akiva movement in Poland until 1947, and was a commander of the Berihah movement until the end of 1947.