Police are facing calls for an independent inquiry after CCTV footage showed a police officer repeatedly punching a woman during an arrest.
Toni Comer, 20, was being arrested last July for damaging a car in Sheffield - an offence she has since admitted.
Her lawyer called for an Independent Police Complaints Commission inquiry.
Police said they would investigate a complaint by Ms Comer but they were "happy" with the officer's conduct. He said he had been trying to subdue her.
The footage, obtained by the Guardian newspaper and shown on BBC Two's Newsnight, shows Ms Comer and a police officer falling down a flight of stairs outside Sheffield's Niche nightclub.
At the bottom, she is restrained by a group of officers, with the officer she fell with punching her five times - although it is not clear on what part of her body.
The footage shows Ms Comer, who is from Sheffield, writhing around on the ground, which she says was due to an epileptic fit. Police say she was trying to resist arrest.
She is later dragged to a police van with her trousers round her knees.
On Wednesday, at Sheffield Magistrates' Court, Ms Comer admitted criminal damage to a car, which she said happened after she was ejected from the nightclub, where she had been drinking brandy and had become aggressive.
She was given a conditional discharge and ordered to pay £250 to the vehicle's owner.
Toni Comer has admitted an offence of criminal damage
Ms Comer says she is now preparing a civil case against the police in relation to her arrest, during which she said she suffered cuts and bruises.
"I couldn't believe it to be honest, I couldn't believe they could do something like that at all," she said.
Her father Leroy Walcott said: "If anybody in any other situation, another male had been hitting a woman or a female, there's no way that would be acceptable."
He added that "it's the message that's actually being sent out".
"I've got a 14-year-old son, he's actually seen this footage. How do I explain to him that this is justified, regarding the police?"
In a statement broadcast by Newsnight, the police officer who punched her, named as Pc Mulhall, said Ms Comer had become violent.
"As her hands became free she tried to grab handfuls of my genitals and knee and kick me in the same place.
"At this point, I struck her as hard as I was physically able with my right fist in an attempt to subdue her. There was no apparent effect so I did this twice more."
He said she continued to resist handcuffs.
"I now struck her as hard as I was physically able in an attempt to deaden her arm...in the end I had to use brute force," he said.
Newsnight showed the footage to former Flying Squad commander John O'Connor.
He said: "The fact that these blows were in pursuance of trying to subdue her to get handcuffs on seems to me to be quite reasonable."
But local race relations manager Ruggie Johnson, who first obtained the CCTV tape, said: "It is absolutely shocking indeed to think that images like these could be coming out of South Yorkshire.
"This is not the deep south, this is South Yorkshire."
Chief Superintendent Ali Dizaei, of the National Black Police Association, also called for an investigation by the IPCC, saying it was a concern that race may have been an issue.
Shami Chakrabarti, director of human rights group Liberty, also demanded an urgent investigation into the officer's actions.
"These images turn the stomach," she said.
South Yorkshire Police said: "The force is outraged at the nature of the report itself, and the possible suggestion that this may be linked to any kind of racist incident.
"We are satisfied with the way this incident was handled and happy with the conduct of the officer but we will investigate the circumstances of the complaint as we would with any complaint.
"It is worth noting that as a result of the original incident the individual was convicted of a criminal offence."