One officer can been seen kneeling by the woman as they get her on her back
Footage has emerged of a police officer punching a woman in the head on a Jersey Shore beach in the US.
The video shows the officer striking the 20-year-old woman, wearing a swimsuit and hotpants, twice after pinning her down on the sand on Saturday.
People can be heard shouting "stop resisting" at the beach in Wildwood, New Jersey.
One officer can been seen kneeling by the woman as they get her on her back while another pulls her legs straight.
A number of officers have been reassigned to administrative duty while the incident is investigated.
Wildwood Police said the suspect - Emily Weinman of Philadelphia - has been charged with underage drinking, two counts of aggravated assault on a police officer, disorderly conduct and resisting arrest.
She is also charged with spitting at an officer.
Police chief Robert Regalbuto said he found the footage of the arrest "alarming" but did not want to "rush to any judgement" until the investigation is completed.
Wildwood mayor Ernie Troiano Jr described the incident as a "shame".
He added that police would soon release body camera footage, which he said would reveal officers were insulted and spat on.
"It wasn't just that this officer decided to beat her up," he told The Philadelphia Inquirer.
"That wasn't the case."
Mr Troiano said he did not know the whole story but added: "We don't like to see anyone get hit, period. But then again, when you have someone who's aggressively attacking you or spitting at you... I wasn't there. I don't know."
The mayor also hit out at underage drinking in Wildwood, saying alcohol is banned in public and on the city's beaches unless an event has received a permit to allow drinking.
This unedited video contains some bad language and scenes some readers may find distressing:
I was sleeping on the beach and I woke up to this.. i can’t believe it.. pic.twitter.com/UJE5Sy7E4G— Lexy (@HewittLexy) May 26, 2018
Police violence in the US has sparked a series of demonstrations across the country over the past few years.
Based on Bureau of Justice statistics, 466 people have been killed by US police so far in 2018.
Last year that figure stood at 1,147.