Thousands of people were watching the finale of the fireworks display along the Benjamin Franklin Parkway around just after 11 p.m. Thursday when someone set off a firecracker, Deputy Mayor Everett Gillison said.
The noise sent several people in the crowd running and a few fell to the ground, he said.
It also caused a scare as some feared gunplay or something else, sparking still more people to run down the street.
"There was a woman ... right in front of me who fell. She was knocked into one of the younger kids running; her cellphone went flying," said Joe Sangine, 42, of West Chester, who had brought his three children to the show.
Tim Scordato, in an email, wrote that he and his girlfriend were seated just north of the Ben Franklin Parkway as the fireworks finale began and saw "100-250 people, running full speed away from something, onto the lawn, and straight for us."
There were no reports of serious injuries or major incidents at the July 4 celebration, which featured a concert by John Mayer, The Roots and other performers. However, some people took to Twitter and other social media to raise concerns that there had been a shooting, although Gillison said that no gun was involved.
No suspect has been identified in the firecracker scare, which occurred near Logan Circle.
At the time, authorities were unsure if the noise stemmed from a motorcycle that had backfired or the fireworks show reverberating off downtown buildings.
SEPTA Transit Police Chief Thomas J. Nestel III tweeted just before 11:30 p.m. that the "Stampede in the 1700 block of Parkway caused by nervous people hearing fireworks reverberate off buildings. No danger!"
Last year, two teens were shot in the leg a few blocks from the parkway during the Fourth of July fireworks.