Johnson

ERIC BAKOVIC does a nice job breaking down why some people might say prótesters and others, protésters. But he skips over an interesting little transatlantic dimension.

Some noun-verb pairs have different stress patterns: you recórd a récord, and a pérmit permíts you to do something. So it is with protest: the verb, the original word, is protést, "to object to something". The origin in the OED is given as "orig. and chiefly Scottish law", by the way, with the earliest citation in 1429. The noun, meaning "objection", appears about half a century later.

The OED gives only one stress pattern for the British pronunciation of the verb: protést. But the American pronunciations are given as protést and prótest both. I think Mr Bakovic is right that the second verb is something that you do as a group, often connected to a political grievance. "The lady doth protést too much" versus, at least in many American mouths, "Libyans are prótesting the 42-year-long rule of their dictator." The second, prótest, is probably back-formed from the noun prótest.

On to "protesters". Agentive nouns usually require just adding the -er suffix, so a blogger is someone who blogs. As for stress, it will be added to the verb (and its stress pattern): recórder, not récorder. So I suspect that this mental rule will, therefore, produce protésters for those who have only protést as a verb (and if the OED is right, this groups would be chiefly British). If you have the verb prótest in your vocabulary, meaning "to express a larger grievance, and especially a political or economic one, usually as part of a group", you'll say prótesters. But if you're in this later group, you probably keep two verbs, not one. "The lady doth protést too much" would call to mind Shakespeare. "The lady doth prótest too much" would describe an activist trying to shut down the local community college for offering insufficient vegan options in the cafeteria.