CHESTER, Pennsylvania -- Police announced an investigation into a St. Patrick's Day celebration that turned violent in Delaware County, Pennsylvania.
Video from a viewer that was sent to sister station WPVI in Philadelphia shows a Chester police officer striking a young woman in the face with a weapon after the woman appears to push him.
Police say they responded to reports of a riot Saturday afternoon just after 5 p.m. in the 900 block of East 16th Street.
Investigators say a violent altercation broke out when a group forced its way into a party inside a home and assaulted several people.
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Moments later, police say 20-year-old Dominique Difiore struck a police sergeant and was taken into custody.
The video has Widener University coeds talking.
"Some people would say it was excessive force, though she did push the officer," Widener senior Zachary Whalen said. "So as soon as she did that, I was like, 'Oh, this is not going to be good.' "He immediately grabbed her threw her to the ground."
Chester police say Difiore struck a police sergeant first.
"I couldn't believe it, a man hit a woman like that," Cameron Thomas said.
In part of a statement released Sunday, Chester Police Department Chief James Nolan said, "A partial video of the event is circulating on various social media platforms. The incident, the video, and level of force used in connection with the event are all currently under investigation."
In addition to Difiore, police arrested 21-year-old Shawn Connelly, 20-year-old Tess Herman and 18-year-old Jaylene Westfall.
Video also shows a second altercation with an officer and another woman. It's unclear if she's one of the four arrested.
Widener University released a statement saying that the four people arrested were not students.
Police are working to determine if anyone else will be charged.
The other part of the investigation involves the officers themselves. The department is looking into their use of force and whether or not it was excessive.