Officers say it happened when police responded to North 22nd Street and Lehigh Avenue at Dobbins High School.
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Police were called to the scene to assist in breaking up a large crowd of juveniles who were fighting.
At its peak, investigators say the crowd may have contained at least 500 people.
While dispersing the crowd, officials say a juvenile wearing a black hoodie, black pants, and a grey backpack ran up from behind a sergeant and kicked him in the back.
The sergeant was subsequently knocked to the ground.
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Once he got up, officials say he saw the suspect running eastbound on Lehigh Avenue toward 21st Street.
Officers were able to stop the suspect on 21st Street, according to police.
As a result of this incident, four people were cited for disorderly conduct and four others were arrested for assault on police.
No information has been released on the identities of the people involved.