An internal investigation was underway Monday after a viral video emerged showing a Pennsylvania police sergeant striking a woman.

The Chester Police Department, in a statement, said it was responding to a call of “what was described to them as a riot” on Saturday afternoon when officers “located a large group of people fighting in the street.

“While police were gaining control of the situation a female identified as Dominique Difiore, 20, struck a police sergeant,” they said. “She was taken into custody by officers on the scene.”

WARNING: GRAPHIC LANGUAGE

The brawl, police said, was linked to a group of people who kicked down the door of a home where a party was happening, after being denied entry. Police added there were “numerous assaults and a college-age St. Patrick's celebration going on in the neighborhood at the time of this incident.”

Video footage shows a woman appearing to take a swing at an officer before he hits her back in response, causing her to fall to the ground.

“I think it was excessive force,” Matt Pfaff, a neighbor, told CBS Philly. “I think it was ridiculous to do something like that, no matter how frustrated you get. They’re trained to do better.”

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Police are warning though that the “partial video of the event [that] is circulating on various social media platforms” doesn’t tell the whole story.

“The incident, the video, and level of force used in connection with the event are all currently under investigation," the Chester Police Department said.