The leader dismissed the demonstrators in the park.
searching “指揮者” on both jisho.org and on google images gives me the impression that it refers specifically to musical conductors most of the time, is that also the case in this sentence?
@ErisCeos
Actually 指揮者 isn’t a bad translation if the subject (i.e. the leader) is the one who orchestrated the demonstration. As you explained, 指揮者 often refers to musical conductors, but the primary definition is “persons who give direct orders” and we say 指揮者 to refer to “leader/commander” in military or security contexts. 指揮官 instead of 指揮者 also works in this context.
But the problem here is that we don’t know from the sentence whether the leader initiated the demonstration or not. The leader could be a high-ranking military officer who seized his country and is trying to oppress his opponents including the demonstrators in the park. In this case, the leader himself did not orchestrate the demonstration, so you cannot use 指揮者. Therefore, リーダー as an English loanword would be a safer choice for translation.
Also, please keep in mind that we definitely prefer 指導者 (pron. しどうしゃ) over 指揮者 (しきしゃ) when it refers to prominent political or religious leaders even if such leaders often agitate people and threaten national security. The difference between leaders of a demonstration as 指揮者 and prominent political/religious leaders as 指導者 is the degree of influence and the duration of power/authority. 指導者 also has the connotation of “admiration from the followers” and “empowerment to the followers.”
One more point I would like to make. Particle で is tricky. 指揮者はその公園でデモ隊を解散させた means the group of demonstrators rallied across main streets and eventually arrived at the park, and then the initiator of the demonstration ordered his followers to dismiss.
If you mean that the demonstration was held in the park from the beginning, 指揮者は公園にいるデモ隊を解散させた is the right translation and you cannot use particle で.