CHICAGO (CBS) — A 12-year-old girl was shot in the back this past weekend amid a fight involving a large crowd of young people downtown, police said.

At 8:40 p.m. Saturday, someone fired a shot in the direction of a group in the 100 block of North Michigan Avenue, near Millennium Park.

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It was Monday when police learned a 12-year-old girl was shot in the back in that incident. She was taken to the University of Chicago’s Comer Children’s Hospital, where she was listed in serious condition.

A total of 21 juveniles were arrested after the large crowds gathered Saturday night.

Around 9 p.m., the large crowd of young people showed up near Michigan Avenue and Washington Street. Police were on crowd control after a fight broke out in the large group.

During the gathering, a 15-year-old boy was also shot in the arm in the 200 block of South Wabash Avenue after bumping into another teen. The boy was taken to Lurie Children’s Hospital in good condition.

Earlier, a Chicago Transit Authority bus driver, while inspecting his bus for damages, was pushed and punched by two people. The driver was taken to Northwestern Memorial Hospital in fair condition.

Of the 21 people arrested in connection with the chaos, at least one — a 15-year-old — is facing a felony battery charge.

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Mayor Lori Lightfoot spoke out Monday on what needs to change, while others have their own solutions.

“We shouldn’t have to get to that point, but if we do, we will,” she said.

Lightfoot said when the chaos erupted one tactic used by police was to push people toward trains to go home from Millennium Park where they were gathered.

Second Ward Ald. Brian Hopkins, who has worked with the Chicago Police Department on Loop crowd control in the past, said officers had intel of social media influencers organizing this as an event or flash mob. CPD would not go into deployment details, but they have had much experience responding to similar crowds as recently as July 4th weekend when officers arrested more than 60 people.

Lightfoot says the real work to fix this problem starts at home.

“These kids have to take responsibility, but I’m going to say the parents have to take responsibility,” she said. “Do you know where your kid is? Are you making sure that you’re talking to your children about how they should act in a large crowd?

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Area Three detectives were investigating all the incidents Monday.

CBS 2 Chicago Staff