Philadelphia police released surveillance footage of a mob of teenagers on the night of Oct. 21. Police say the crowd attacked random pedestrians, including Temple University students and a police officer. (Philadelphia Police)

It started with an Instagram post. It ended with four arrests, at least eight injuries and a major security ramp-up at Temple University.

On Friday evening, dozens of teenagers began to gather outside the Pearl Theater in North Philadelphia, brought together by what police said were calls on social media to meet for a 6:45 p.m. showing of the movie “Ouija: Origin of Evil.”

Word spread. By 8 p.m., more than 150 youths had amassed at the intersection near the theater, located at the southern end of Temple’s campus.

Then things turned violent.

As the crowd moved down the street, several smaller groups of youths started attacking random pedestrians, many of them university students who were returning to campus from a Temple football game across town, according to the university’s independent student paper. The teenagers punched, kicked and tackled people indiscriminately, police said, fanning out into the North Philadelphia neighborhood and assaulting passersby over the next few hours.

As of Monday, police had taken four kids between 14 and 17 years old into custody in connection with the attacks, which authorities compared to “flash mobs” that have cropped up in the city in recent years, according to the Philadelphia Inquirer. Authorities said they’re still looking for other suspects.

Joe Lauletta of Holland, Penn., said his 19-year-old daughter was badly beaten after the mob broke out. In a Facebook post that went viral, he wrote that she had just emerged from the subway after watching the Temple football game when a group of 30 to 40 teenagers swarmed around her.

“Every part of her body is badly bruised,” Lauletta wrote. “It makes me cry just thinking about it.”

In an interview later with WTXF, he said his daughter was released from the hospital with no broken bones but still had cuts and bruises covering her body.

“She went down in the fetal position and then they pinned her,” he said. “Her back is really, really cut up.”

Police said two of the people who attacked Lauletta’s daughter were arrested and charged with aggravated assault, robbery and criminal conspiracy, as the Inquirer reported.

On Tuesday, police released surveillance video from a pizza shop that captured part of the chaotic scene, describing the suspects as “approximately 20 black juvenile males and females.”

The grainy black-and-white footage shows several dozen young people walking down the street near the theater when suddenly a fight appears to break out. A group of people swarm around a person wearing a light-colored shirt, pushing, kicking and throwing punches. As the victim is thrown to the ground, some in the crowd gather to watch, while others scramble back and forth across the street. One woman can be seen in the video appearing to record the incident from her phone.

After about 20 seconds, the fight appears to clear and people disperse as a line of cars drives down the street. Police arrive, and an officer is seen talking with someone in the pizza shop, who gestures in the direction of the mob.

Another Temple student said she was in the area when the crowd got out of control and was beaten by a group of kids who “poured around the corner” as she was crossing the street. The student, who didn’t give her name, shared pictures of her two black eyes with the Tab and told the site that she couldn’t open her jaw all the way after the attack. She also said her hair had been ripped out and that her hand and wrist were swollen.

“I remember shoes coming for my face and after that I heard other kids from the group saying ‘Yo chill, yo chill it’s just a girl’ and they pulled my attackers off me,” she said.

As the crowd spread into the surrounding area, a 15-year-old boy threw a police officer to the ground as she was trying to arrest him, causing her to fall on her patrol bike, an NBC affiliate reported. The teen was arrested a block away. Minutes later, another teenager approached an equine officer who had arrived to break up the crowd and punched the officer’s horse in the head twice. He was arrested and charged with assaulting a police service animal, the Inquirer reported.

All told, at least six Temple students and two police officers were attacked, according to ABC.

Temple tweeted out a security alert for “increased police activity” around 9:30 p.m., but some people said it was too little, too late.

TU Alert: Increased police activity due to large groups of juveniles along Broad Street on/near Main Campus. Police responding. Avoid area. — Temple University (@TempleUniv) October 22, 2016

. @TempleUniv I got this TU alert 2 hrs after I was (& many other people were) attacked. Please do a better job of alerting when it happens — Milanka Bee (@milankabee) October 22, 2016

The university said in a statement that it would “nearly double” campus police presence over the weekend and during Halloween, and called on city and transit police to step up security.

“I know that random acts like these are unsettling,” the university’s chancellor said. “While last weekend’s events received tremendous media attention, Temple remains a very safe campus.”

Rumors circulated in the days following the incident that the attacks may have been racially motivated, with some people claiming on social media that black youths were deliberately targeting white people. Police officials said those rumors were false, stressing that the attacks were random.

“There hasn’t been any indication that race is involved in this in any shape or form,” Philadelphia Police Lt. John Stanford told the New York Post.

“Those kids that are actually assaulting people and robbing people, there’s a bigger issue that certainly goes beyond race,” he said. “Unfortunately, perception is reality for a lot of people, but that doesn’t mean it’s factual.”

Police have blamed “flash mobs” for similar scenes of violence in Philadelphia in recent years. In 2013, a crowd of some 200 teens attacked people in the city’s downtown, leading police to arrest 14 people. As early as 2010, police and city officials said they would crack down on flash mobs that had gotten out of control, at one point considering eliminating free transit passes for students in the evening hours in hopes of preventing teenagers from riding public transportation downtown.

“This is bad decision-making by a small group of young people who are doing silly but dangerous stuff,” then-Mayor Michael A. Nutter told the New York Times at the time. “We intend to do something about it immediately.”

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