People gather during last year's Occupy Hong Kong protests. Photo via Flickr user johnlsl
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Socialists in Union Square, New York City, on May Day, 1912. Photo via Wikimedia Commons
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Protesters in Cairo's Tahrir Square in 2011. Photo via Flickr user Ramy Raoof
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Alexander Berkman speaks in Union Square on May Day, 1914. Photo via Wikimedia Commons