A police officer hit a prisoner with "as much force as he could" even though the man was being restrained, a court was told.
PC Paul Ions lost his temper after being struck by Paul Longstaff while he was taking him into the custody suite at Hartlepool police station.
He was captured on CCTV swinging a punch at Longstaff while he was held by officers.
Prosecuting, Tom Moran told Teesside Crown Court: "The respondent's case is that he was restrained and posed no threat."
PC Ions claims he was still under attack and thought he needed to defend himself.
The 29-year-old officer, of Ingleby Barwick, near Stockton, who is suspended by Cleveland Police, is bidding to overturn convictions for assaulting Mr Longstaff and another man, Adnan Asif.
He was jailed for four months in November last year for the assaults, on October 12 and September 27, 2003, but bailed after immediately launching an appeal.
The appeal hearing continues.
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