A King or a Leader

The Talmud (Sotah 41a) teaches that at the Hakhel gathering the Jewish king would read from the Torah. It is not clear to me if this is a Law transmitted to Moshe at Mount Sinai (and not transcribed in the Torah), that specifically the king [is obligated to read], and if there is no king there is no commandment of Hakhel. If this were the case, until the time of King Shaul the Hakhel ceremony was not performed!

However, it is possible that [the mitzva of reading] is not incumbent on the king, but rather on the leader of the generation. The commandment obligates him if there is no king. And so it seems logically.

Minchas Chinuch Mitzvah 612

Excerpt from Hakhel: The Inside Story
Published by Sichos in English