The Great Meeting was a demonstration of 150,000 supporters of the Union cause.
The flag that was fired upon at Fort Sumter was placed on the Washington statue

INSTALLED 2002

At the beginning of the civil war in the United States, after the inauguration of Abraham Lincoln and the bombardment of Port Sumter by the Confederate forces, the great meeting was held on April 20, 1861 in Union Square to support the Union.
The flag from Fort Sumter was raised on the statue of George Washington.

The plaque is an exact replica of the May 4 photograph of Harpers’ Weekly drawn by Homer, Winslow, and is now in the public library of New York. The image presented here is taken from Wikipedia Click for a larger image

Part of a series of plaques surrounding Union Park and describing the history of the square and events from the United Workers’ Movement (Union)

The plaques are made of bronze, designed by Gregg LeFevre and set up in 2002

Additional plaques around the square: Click for cover's details, Click for cover's details, Click for cover's details, Click for cover's details, Click for cover's details, Click for cover's details, Click for cover's details, Click for cover's details, Click for cover's details, Click for cover's details, Click for cover's details, Click for cover's details, Click for cover's details, Click for cover's details, Click for cover's details, Click for cover's details, Click for cover's details, Click for cover's details, Click for cover's details, Click for cover's details, Click for cover's details

More details can be found here Click for the item in The ultimate street signs site