Officials later said that seven people were taken to hospitals with non-life-threatening injuries apparently suffered in the panic.
Police said officers were sent to the circle for a report of a man with a gun. Around that time, a loud noise was heard in the area, police said. Police said they were trying to learn the source of the noise.
A brief video posted on Twitter shows people trying to leave the circle hurriedly. At first they appear to be obstructed by linked metal crowd control barriers that ran around the perimeter. Some climb the barriers. As they do, the barriers topple, with a clanking, crashing sound.
It was not clear if the noise made by the falling barriers was taken to be the sound of gunshots.
At the circle, a man was arrested and a weapon was found, police said.
As officers arrived, people were fleeing, according to police.
Lindsey Migliore, a physician, said that as she was taking part in the parade “everyone started running behind us, running and screaming,” heading away from the circle. She said she helped a woman who had apparently twisted her ankle.
The “entire crowd dispersed and everybody started running,” said Jason Lindsay, another witness.
The incident, which occurred about 7:20 p.m., ended the parade, officials said. It was not clear when the parade was scheduled to end.