TAMPA — The security video shows a young man in a yellow jail uniform virtually disappearing under the punches and kicks of cellmates. Polk County deputies rush into the frame, one wielding a stream of pepper spray. An attorney hits pause at the end of the taped melee, freezing it as two boys writhe on the floor. The seconds-long footage captured at the Polk County Jail opened the first day of testimony Monday in a federal lawsuit against the Polk County Sheriff's Office and Corizon, a company it contracts to provide inmates with medical care.The young man attacked in the video, now in an adult's orange jail uniform, took the stand to describe the scene for Southern Poverty Law Center attorneys.The nonprofit civil rights group filed the suit, now a class action, on behalf of seven juveniles. They allege overly harsh conditions at the jail caused by poor supervision, inadequate rehabilitative programs and an overuse of pepper spray. The jail typically houses 60 to 70 juveniles.Polk Sheriff Grady Judd, who has denied the allegations, defends his use of pepper spray as a last resort for juveniles who refuse to obey commands. He credits it with ending confrontations before they turn physical.But Miriam Haskell, a Southern Poverty attorney, told U.S. District Judge Steven Merryday in her opening statement the chemical only perpetuates violence. "It's used at the jail to threaten and instill fear," Haskell said.Jonathan Trohn, a Lakeland lawyer representing the sheriff, argued the center's accusations aren't constitutional violations as alleged. Juveniles rarely received "objectively serious" injuries at the jail, he told the judge, and pepper spray is used "little more than three times a month."The judge must consider the predisposition of the juveniles, Trohn argued, showing a picture of a shank a fight-prone boy had made. If the judge thinks jailhouse fights and suicide attempts were caused by the sheriff's policies, he said, "you're not seeing the whole picture."Merryday ordered the attorneys to refer to children involved in the case by their initials only because they were minors when incidents occurred.Before the fight, captured on video, J.P., the boy attacked, was gathering laundry. He testified other juveniles told him they didn't like boys from his town. He later learned boys in another dorm communicated to his cellmates with hand signals: Attack J.P., and we'll give you cookies and candy from the jail canteen.In the video, a boy drops J.P. with a punch. Another lifts him up to what appears waist level and then drops him. Boys circle, landing blows. J.P. testified that no deputies were in sight when the attack broke out. Deputies then rushed in and one immediately fired a shot of pepper spray.J.P. testified the deputy sprayed him at close range, even as he was on the ground. He told staff the deputies reacted appropriately during the Jan. 22, 2012, incident but later changed his mind. By the time deputies intervened, he was fighting with one other cellmate. "It was me and another juvenile," he said, "so they could have pulled us apart."Testimony continues today.