The City of Chicago used heavy municipal trucks and police cars on Friday night to stop a large group of teenagers from taking over a South Side beach.
Bright blue police lights flashed in the evening air near the completely blocked entrances of Rainbow Beach.
Big city salt trucks from the Department of Streets and Sanitation parked bumper-to-bumper with marked police cars so no one could drive into the recreation area.
The police department took this extreme step because a viral post on Facebook was telling hundreds of teens to meet at the beach at 5:00 p.m.

Here is the reality facing families living on the South Side as the warm summer weather brings new safety fears to the lakefront.
The sand and parking lots stayed totally empty at 5:45 p.m. because no large crowds showed up behind the heavy city trucks.
The city blocked the beach gates early because of the terrible fights and shootings that shook local neighborhoods just a few days ago.
The new security plan aims to break up these unapproved social media flash mobs before they can even start.
The University of Chicago Police Department also sent an urgent warning to students and neighbors on Friday afternoon.

Campus police warned that groups of young people might try to start a secondary meetup near 52nd Street and Lake Park Avenue.
Mayor Brandon Johnson spoke directly to local families on Friday to ask for immediate help with this growing city problem.
The mayor asked parents and guardians to find out exactly where their kids are so everyone stays out of trouble this weekend.
“As we enter the weekend, I am urging parents, guardians and trusted adults to help prevent unsanctioned teen gatherings, know where your children are, and talk with them about the risks of attending these events,” Mayor Johnson said during his official statement.
The city leader also shared a very firm warning for young people looking at the party invitations on their smartphones.
“To Chicago’s young people: do not attend these gatherings,” Mayor Johnson said while explaining that officers will strictly enforce the night curfew.
City Hall intends to use both police units and community violence safety workers to talk with kids on the streets and keep things peaceful.

The heavy police presence shows that city leaders are very worried about a repeat of the chaotic Memorial Day weekend.
A routine crowd control call on the Near West Side turned into a direct attack on law enforcement last Sunday.
An 18-year-old man drove his vehicle straight into a line of officers in the 1200 block of South Loomis Street as they tried to move people along.
Five Chicago police officers had to go to the hospital with painful injuries from that vehicle attack.
The scene told a different but equally dangerous story on Monday night near the popular lakefront in Hyde Park.
Several hundred teenagers swarmed the historic neighborhood near the Museum of Science and Industry and started fighting.
Dozens of kids jumped on top of blue-and-white squad cars to stomp out the roofs while others threw heavy objects at retreating officers.
The scary night turned deadly when gunshots rang out after the massive crowd walked away from the 57th Street Beach.
Three young people collapsed onto the pavement with gunshot wounds near 55th Street and Cornell Avenue after crossing DuSable Lake Shore Drive.
Responding officers arrested 53 people and took away nine hidden firearms during the chaotic aftermath in Hyde Park.
The bloodshed on the lakefront highways forced tactical units to change their safety plans completely for the rest of the summer.
People who live near these beautiful parks can protect their own homes by watching the streets and checking local neighborhood apps.
Safety experts say you should keep your personal vehicles locked inside secure garages and stay away from beach corridors after the city curfew kicks in.
The giant salt trucks will keep blocking the entrance to Rainbow Beach until the online invitations fade away completely.
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