Police in Philadelphia have launched an investigation after a video was posted on social media apparently showing an officer beating a teenage girl.

Police were called to break up a street fight between two groups in the south-west of the city on Monday (2 January).

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A video posted on Instagram shows the groups fighting, and a police officer pulls a teenage girl away and pushes her back. The camera then pans away, and when it returns the police officer throws the girl to the ground and while on top of her swings blows at her.

The officer then pulls the girl up by her hair, and the video ends.

Takeema Bundy, a friend of the girl, told NBC10 "She's 16 years old, I don't think she deserves that. The cop shouldn't have done that. That lady was like 30- something."

The teenager, who has not been named, is currently in police custody.

Philadelphia Police say they're aware of it and its being investigated by the Internal Affairs Division.