The Independent Police Complaints Commission has launched an investigation after a police officer used "brute force" when he struck a young black mother-of-one while arresting her.
PC Anthony Mulhall, of South Yorkshire Police, was removed from frontline duties after it emerged he hit Toni Comer in order to subdue her so she could be handcuffed.
The incident, captured on CCTV, occurred outside the Niche nightclub in Sheffield last July and 5ft 6in Ms Comer, who weighs about nine stone, is pursuing a civil action against the force.
Nicholas Long, IPCC commissioner for Yorkshire and Humberside, said: "I have seen the CCTV footage and decided it is in the public interest for a wholly independent investigation to be conducted.
"This investigation will look into the circumstances of the incident to determine whether the force used by the officer when making the arrest was justified and proportionate."
In a statement made after the incident PC Mulhall said he hit her "as hard as I was physically able".
Speaking at her parents' house in Erskine Crescent, Arbourthorne, Sheffield, Ms Comer, 20, said she was shocked by the footage.
"I was very shocked and very upset that they would do this to somebody. I didn't think it was possible," she said.
"I thought police arrested people for doing things like that, not that they did it themselves. I would like to see some kind of justice so this never happens to anybody else in the future."
Ms Comer said she had no memory of the incident, but that she suffered bruising to her arm, back of neck, above her eye, and to the side of her head. She is epileptic and believes her movements may have been caused by a fit.
