The hearing at Aberdeen Sheriff Court was told how Constable Colin Leask, a 29-year-old martial arts expert, kicked 19-year-old Kevin McGeachy in the groin before launching a flurry of blows to the suspect's head and finally throwing him to the ground.
Leask, who took up karate and martial arts when he was a 12-year-old, had denied the assault which took place last July. He showed no emotion at the verdict.
The court was told that Mr McGeachy had been arrested for drugs offences and then taken to Grampian police headquarters in Aberdeen for a strip search. Other officers in the cells witnessed Leask carrying out the attack which left the teenager lying on the ground bleeding heavily from the head.
Giving evidence on his own behalf yesterday Leask, who weighs 18 stone, claimed he had only used self-defence techniques on Mr McGeachy because he feared he was about to be attacked. Leask, who gave a demonstration in court of the martial arts skills he claims to have used on Mr McGeachy, said his punching power amounted to three times his body weight and could cause serious injuries to opponents.
The court was also told how Leask had taken part in national and international martial arts competitions and turned down the chance to box at the Commonwealth Games.
Sheriff Graeme Warner said the charge was a serious matter and deferred sentence until 15 April for social inquiry and community service reports.Reuse content