The question occurred to me while reading the Bible, but is perfectly secular in nature, so please do not turn this thread into another (anti-)religious flame war. Thank you.
In the Bible, you can find many accounts of dialogue between a single person and a large group, like "He said unto them: ..... , and they said unto him: ....". The "he" (and perhaps occasionally "she") being various key persons from Abraham to Pontius Pilate to some of the the Apostles, and the "them" being sometimes a whole city or even a tribe. In some cases, the dialogue occurs in a very tense situation, with the "he" being in serious danger of getting killed by "them". I will look up some specific examples if you wish.
I suppose similar scenes can be found in other old texts, it's just that the Bible is what I am familiar with.
Now my question is this: how was it possible for a single person to make himself heard by so many, when they were not even favourably disposed to listen to him, at a time when amplifiers and loudspeakers weren't even being dreamed of?