A man was wounded in the shooting Tuesday night and three Chicago police officers were injured while breaking up a fight in nearby Old Town.
During a press conference Wednesday afternoon, CPD Superintendent David Brown said groups of young people began showing up to the beach about 4 p.m. Tuesday, responding to a "flyer" posted online, encouraging people to come to the beach.
WATCH: CPD Supt. Brown gives update on North Ave. Beach, CTA shootings
CPD made 2 gun arrests between 5 and 6 p.m., finding firearms during bag checks, Brown said.
A 20 and 21 year old were arrested.Just after 9:45 p.m., Chicago police said a 19-year-old man was outside at North Avenue Beach when he was shot multiple times by another man.
Video released by Chicago police shows a man sprinting down North Avenue Beach; police said he took off running seconds after the shooting.
In the video the man is shown pausing and tossing a bag before continuing on.
"One of the two tossed a fanny pack which was recovered, and a Glock handgun with an extended magazine was recovered from that bag," CPD Supt. David Brown said.
The victim was taken to Illinois Masonic Medical Center in critical condition, where he has since been stabilized.
According to police, the one man was taken into custody by responding officers. But the man who tossed the fanny pack is still on the run, and police hope someone recognizes him from the newly released video.
Police said they were tipped off about of a gathering at the beach and sent extra officers, but to no avail.
Alderman Michele Smith, whose 43rd Ward includes North Avenue Beach, said hundreds of people on the beach were drawn there by social media for what she calls an illegal gathering.
"We are really outraged," she said. "I have lived here for 44 years and having guns at our beaches is absolutely unacceptable."
Alderman Smith said she wants the city to hire private security, establish a security perimeter around the beach and install metal detectors.
Three young women who were at the beach said a few fights had broken out before the shooting happened. They heard someone say, "Somebody got a gun, somebody got a gun" before hearing at least six shots, according to the women, who asked not to be named.
"We just heard shots and ran," one of the women said.
"Just crazy. People throwing each other everywhere, people running. It was nuts," said witness Dan Brannagan.
"Our manager was like, 'Alright, everybody has to stay inside, nobody can get out and nobody can leave,'" said Cesar Herrera, witness.
Meanwhile, while crowds were dispersing the beach Tuesday night, a fight broke out at North Avenue and Wells Street, about 10:50 p.m.
Three officers were injured there, and a 16-year-old girl, and two 15-year-old boys were taken into custody.
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One officer suffered scrapes and lacerations to the arms and was treated and released on the scene.
A second officer suffered swelling to the right eye and was also treated and released on the scene.
And a third officer, who was hit with an unidentified object to the back of the head, was taken to a local hospital, where he was also treated and released.
Sun-Times Media contributed to this report.