Girl arrested
Video of the weekend incident appears to show a San Antonio police officer punching a 14-year-old girl in the face Youtube/Screenshot

The San Antonio Police Department (SAPD) has launched an investigation after a video posted on social media shows an officer appearing to punch a 14-year-old girl in the face before arresting her.

The viral video shows the forcible arrest of the teen, who was attending a 15th birthday party known as a quinceañera on Saturday (20 May).

Authorities were called to the birthday party after two adult male attendees got into an altercation.

A preliminary police report provided to the Huffington Post by the SAPD states one of the men accused the other of becoming "inappropriate with the juveniles".

The party was then kicked out of the facility where the celebration was being held. Fighting between the men continued and authorities were called.

The 39-second video, shot by a bystander, shows between three or four San Antonio police officers arresting the girl as her mother screams in the background, trying to help her. An officer can be seen diving towards the girl before her head snaps around and she falls backward.

The minor was arrested on Saturday on a criminal charge of assaulting a public servant, according to the San Antonio Express-News.

The girl remained in Bear County Juvenile Detention Center until she was released by Judge Arcelia Trevino on Monday (22 May). The girl's attorney, Artessia House, said her client was an honour-roll student with no history of violence. House maintained that the girl did not assault the officer and that the video shows an officer "clear as day" hitting the girl.

The officer "lacked the temperament and the restraint when it came to dealing with this child to the point where he became an aggressor," House told HuffPost. "When we talk about use of force, the force has to be proportional. ...That force does not seem that it would be proportional—even if the child did hit him."

"Because she is a 14-year-old minor, she needs to be treated as a 14-year-old minor—not as an adult," she added.

House told HuffPost that the girl did not receive medical attention at the scene. She was able to see a doctor at the juvenile detention centre but was not given an ice pack when she asked for one. The attorney said she expects the charges against the girl will be dropped.

"Per SAPD policy, the events leading up to the arrest of the juvenile for assaulting a public official is being investigated to ensure compliance with Department policies," San Antonio Police Chief William McManus said in a statement.

San Antonio Mayor Ivy Taylor called the video of the incident "hard to watch and to listen to". Taylor added: "SAPD is now reviewing body cam video to determine exactly what happened and Chief McManus is keeping me updated on the ongoing investigation. When I supported funding for body cameras it was for instances like this so that we can see exactly what happened and protect officers and citizens."

According to the Express-News, the video was removed from Facebook but remains on YouTube and Twitter.