The 19-year-old seems limp, and may even have been unconscious. As the officers struggle to pick her up and drag her to a waiting police van, the teenager's trousers fall round her ankles.

Toni Comer remembers nothing of the incident outside the Niche nightclub in Sheffield. She is an epileptic and thinks she had a fit. But the footage, which has been obtained by the Guardian, is now the centre of a police inquiry that will test whether the officers involved acted proportionately during the arrest, or with unnecessary force. The video, which can be seen on the Guardian Unlimited website, has already caused concern in Sheffield among those who have seen it.

Ms Comer's lawyers are demanding that the Independent Police Complaints Commission launches an investigation to look properly at the inconsistencies they believe are found in a statement made by PC Anthony Mulhall. The officer has admitted he hit her but claimed he acted in self-defence and only after extreme provocation.

The struggle involving Ms Comer, now 20, from Sheffield, took place when she was arrested outside the club last July. She had earlier been ejected from the club and had then damaged a car belonging to a security guard in the club's car park.

Yesterday she pleaded guilty to criminal damage and was given a conditional discharge and ordered to pay £250 compensation to the car owner. Sheffield magistrates court was told by Danny Simpson, Ms Comer's lawyer, that the arrest had caused "a great deal of distress" and an officer "punched Ms Comer five or so times until she would say she was unconscious". He added that it was now the subject of a complaint to the police and "may in due course be the subject of civil proceedings".

By chance, the club's security cameras caught the arrest. The film shows Ms Comer and PC Mulhall at the top of a fire escape. During a struggle, they roll down the stairs until PC Mulhall lands on top of her. He can be seen striking her five times. Other police officers arrive, including a dog-handler and dog, and she is dragged to a police van with her trousers down.

Ms Comer admits she had been drinking brandy and had become aggressive in the club. She is convinced she had an epileptic fit and the officer has said he saw foam at her mouth. She has suffered more than 20 such fits, the first when she was pregnant with her two-year-old son.

She said she was unaware of what had happened during the arrest until shown the film. "I was shocked when I first saw the film. I didn't think they'd do something like that. I didn't want to go out of the house after seeing it."

She acknowledges the police would have been unaware of her medical condition and says that her father has told her that she can be aggressive during a fit. "But they should be aware of those types of illnesses. It's shocking what the police did to me - I wouldn't wish it on anyone."

Her father, Leroy Walcott, said yesterday that he had been shocked by the film. "To see that happening to your daughter is bad enough but the fact that police officers are involved makes it doubly worse."

In a statement after the arrest, PC Mulhall acknowledges he struck Ms Comer but said he only did so in self-defence and after extreme provocation. "She began to kick, spit and made attempts to bite me," he said in his statement. "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 able with my right fist in an attempt to subdue her. There was no apparent effect so I did this twice more."

When she kept resisting, he said, he hit her again. Trying to put her in handcuffs, he states that "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."

Ms Comer, who is 5ft 6in tall and weighs nine stone, has one previous conviction for handling when she was 14. She was given community service.

Ruggie Johnson, coordinator of The Monitoring Group North and editor of the local magazine Wha' a gwan, first obtained the footage. "I was absolutely shocked when I saw it. It's like something out of the Deep South." He said it reminded him of the Rodney King episode in Los Angeles when police officers were seen on camera making an arrest of a drunk driver.

A spokeswoman for the South Yorkshire force said: "A complaint has been lodged and is now being looked into."

Peter Mahy, the lawyer handling the complaint said: "It raises very serious concerns, hence my call for it to be investigated independently by the Independent Police Complaints Commission."