View of the George Washington Equestrian Statue in Union Square on the Year of its Dedication in New York City (1856)

View of the George Washington equestrian statue on the year of its dedication in Union Square in New York City, New York, 1856. By the Langenheim brothers.

Source.

Two Views of the Great Sumter Rally in Union Square, New York City (1861)

Animated stereoview of the crowd in Union Square during the Great Sumter Rally. Source.

View taken after the end of the event after much of the crowd had dispersed. From Miller's Photographic History of the Civil War.

Two photographs of a large crowd of people in a gathering to show support for the preservation of the Union after the fall of Fort Sumter in Union Square, New York City, 1861. Around 100,000 people were said to have attended the event which would have made it the largest gathering of people held in the United States up to that point. It succeeded in raising money and enlisting soldiers for the early Union Army and was credited with helping to defend Washington, D.C., in the early months of the war. The event has variety of different names including "The Great Union Meeting," "The Great Mass Meeting," and the "Great Sumter Rally."

More information.
Original New York Times Article

View of President Abraham Lincoln's Funeral Procession on Broadway Entering Union Square in New York City (1865)

View of President Abraham Lincoln's funeral procession on Broadway entering Union Square in New York City, April 1865. The house on the left of the second photograph is where a young Teddy Roosevelt can be seen in a window (just out of frame here) of another photograph which was taken from the same position. Animated stereoview.

Source.
Teddy Roosevelt photograph article link.