Police are under orders to arrest anyone on the Mag Mile engaging in criminal behavior after a large group of teens wreaked havoc in the area over the weekend, an alderman said.

Alderman Brian Hopkins, whose ward covers part of the Mag Mile, confirmed the information to NBC 5. Not only that, but dramatic cellphone video shows a security guard being assaulted by two people outside the McDonald’s at Chicago Avenue and State Street. It's unclear if the incident is related to the weekend chaos. The off duty suburban police officer was able to break free and pulled out a gun on his attackers who ultimately flee, the video shows. It’s one of several mob like incidents across the Mag Mile and River North on Saturday.

"There were several hours where it was just mayhem down here," Hopkins said.

The 2nd Ward alderman said the incidents included people being punched and pushed to the ground. Police arrested 21 people— including 20 juveniles.

"This is a serious problem we have to stop it, we have a limited number of tools in our toolbox," Hopkins said. "Arrest is one of the things we can do, so we’re doing it."

There was a show of force by police outside the McDonald's on Monday where the guard was attacked.

"We are disturbed by this situation," the fast-food restaurant said in a statement. "We will continue to take appropriate measures to provide a safe and enjoyable experience for our customers."

In another incident, teens swarmed a yogurt shop, made a mess and tried to leave without paying.

"This is supposed to be best neighborhood in Chicago," resident Sallie Kraus said. "(It) doesn’t bode well for the summer season."

Water Tower Place instituted a policy a while back that teenagers must be accompanied by an adult after 4 p.m on Fridays and Saturdays. That policy is working, officials say, and now officials are focusing their attention on the street.