Depending on whom you read, there are protesters or protestors aplenty demonstrating against real and perceived injustices. Mr. Rewrite is somewhat perplexed that "protestors" has about 5 percent of the play in articles indexed by Google News. Here's why ...
Some dictionaries, including Merriam-Webster OnLine, accept "protestors" as a variant. But that's a stretch. You'll want to go with "protesters" because that's Associated Press style and the advice you'll get from any dictionary or usage guide.
Mr. Rewrite's favorite book, Garner's Modern American Usage, notes that there's only one sense in which protester is preferred, and it's capitalized in that case: "a Scottish Presbyterian who protested against the union with the Royalists in 1650."
The following examples are from outfits bound by AP style and most likely not writing about surly 350-year-old Scottish Presbyterians: