Police on Saturday responded to calls of a fight between two men at a quinceanera — a traditional Hispanic celebration of a girl's 15th birthday.
After the altercation, police arrested the 14-year-old — who was unidentified — on a charge of assaulting a public official.
However, a video taken by an eyewitness at the party appeared to show one of the officers striking the girl as two other cops appeared to forcibly restrain her.
Her mother, who also appeared to be restrained by officers, was heard screaming.
Police are now investigating the events leading to the teenager's arrest.
In a statement Monday night, SAPD Chief William McManus said, "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."