Philadelphia Police descended onto City Hall Wednesday afternoon to break up another flash mob.
A handful of Center City merchants called police to report a large group of teens causing a disturbance near Broad and Chestnut Streets around 4 p.m., Lt. Frank Vanore said.
That disturbance eventually grew into a fight which moved up towards Market Street, police said.
Dozens of teens could be seen running west along Market Street -- darting in and out of traffic and jumping over security barriers to get away from pursuing officers.
The group then made their way west across the city hall grounds to Dilworth Plaza, where they ran underground into the SEPTA station.
Officers say they eventually caught up to 22 people believed to be involved. They later placed 18 people under arrest.
One of the teens was charged with assaulting a police officer, police said.
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"One highway unit did receive some injuries from a elbow," Lt. Vanore said. "The majority of the people will be charged with riot and disorderly conduct."
Philadelphia Police have increased patrols in the area of Market East after a group of almost 100 teens caused chaos along Market Street in February -- pushing down pedestrians, fighting on street corners and vandalizing Macy's Center City.
"We just can't tolerate this sort of stuff and we're not gonna tolerate it," Police Comm. Charles Ramsey said.