Or external agitators -- just not exterior, which is typically used for the outside of buildings or other structures.
I also agree that 'prod,' with or without 'on,' isn't very idiomatic. Better options might include 'spurred on' or 'goaded,' if you use people as the subject. But if you use 'fervor' as the subject, you need a different verb.
Local residents were spurred on / goaded into action by external agitators.
Their fervor was stirred up / heightened by outside agitators.
Also, 'so' here is not a true causal connector; there's no direct logical relation between the two unsupported statements of opinion. Perhaps you mean something like 'For that reason it seems likely that they were stirred up by outsiders.'
(Actually, if this is for an academic essay or for journalism, you would need to give evidence for both assertions -- that local residents were incapable of, or uninterested in, protesting on their own behalf, and that outside agitators were in fact present. I must say I'm skeptical on the face of it, as those kinds of claims are typical of authoritarian governments.)