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The Industrial Workers of the World gather in the North end of the square in 1914.
An undated look South from Union Square. Notice how few places there are for groups to gather.
Union Square as it appeared in 1850.
An illustration of the 1886 Workingman's Parade
Workers march on May Day, 1913.
"60,000 communists gather together in New York City to protest against unemployment and capitalism" in 1934
"Communist demonstrations 1934"
A May Day immigration protest in 2010.
Just for context, this was a Columbus Day Parade in Union Square in 1892.
A sign advertising the planned General Strike for May Day, 2012 (Scoboco's flickr).