A police officer has been found guilty of assaulting a vulnerable teenage boy after arresting him under the Terrorism Act.

Inspector Dean Gittoes, 49, of Oak Tree Rise, Merthyr Tydfil, “lost all rational thought” when he “unlawfully” detained the 16-year-old outside Merthyr Tydfil Police Station on August 20 last year.

The incident was captured on a now-deleted YouTube video that was recorded by the youth, who claimed at the time to be “auditing” the station in South Wales.

The term auditing refers to a global online community of people who record and upload videos of government buildings, such as police stations.

Prosecutors said the arrest was “unlawful”, claiming Gittoes, who was off-duty at the time, did not honestly believe the youth was committing a terrorism offence.

Instead, they claimed, Gittoes allowed his judgment to be “clouded” by his dislike of auditors and his ongoing frustration with his bosses over how a similar incident had been handled.

Christopher Rees, defending, said the veteran officer had genuinely believed the boy to be involved in a hostile reconnaissance of the building and that any footage he uploaded to the internet could have assisted terrorists.

The court heard how guidance issued by South Wales Police and seen by Gittoes, after similar incidents across the force area, advised officers that members of the public have a general right to film government buildings on public property.

It also said that if a person identifies themselves as an auditor, to ignore them.

Footage played to Gwent Magistrates’ Court showed Gittoes in a Swansea City football club shirt and black shorts confronting the teenager seconds before grabbing the boy’s phone, putting him into an arm lock and leading him into the custody suite.

The youth’s distressed cries of pain and “he’s choking me” can be heard, while Gittoes’ is heard telling him to “stop struggling”, adding: “You’re a clever little internet freak who’s about to learn the hard way.”

Convicting Gittoes, District Judge Sophie Toms called the incident a “continued unlawful assault against a vulnerable 16-year-old boy”.

Judge Toms said: “What is absolutely apparent is that you were frustrated with nothing being done to stop the filming of the police station, that senior officers were not taking it seriously and were not supportive.

“Your view was that it was a dangerous situation as videos could be used by terrorists, but I haven’t seen any evidence to that affect.

“The best evidence of your thought process comes from the youth’s phone when it was recording in your pocket, you having left the youth in the custody unit and forgotten the phone was still recording.”

The judge read out the conversation between Gittoes and his colleagues when he said: “Six weeks ago I dealt with someone like this and the bosses so far think it’s a joke.

“Anyone I catch now I don’t give them a second chance.

“I’ve got 36,000 people on the internet calling me a shit.

“I thought f*** him this time.

“I’ll do what I should’ve done last time.”

He also spoke about having had a bad weekend before calling the youth “a clown”.

The incident he was referring to had happened weeks before with another member of the auditing community and was the subject of a public complaint at the time.

A video of the interaction was uploaded to the internet and attracted thousands and views and comments, some of which discussed finding where the inspector and his family lived.

Gittoes said during the trial that in hindsight he had regretted not arresting the auditor.

Returning to his arrest of the youth, Judge Toms said: “You were clearly agitated and took no more than 30 seconds before you laid your hands on him.

“It was a snapshot decision, and it was evident you were unable to control yourself or your actions.

“Significant unnecessary force was then used against this boy when he was offering no resistance.

“You grabbed his arm, used force against him, marching him into the station, twisting his wrist, causing him to fall to the floor, pushing him against the wall, grabbing the back of his hoodie preventing him breathing properly.

“He was crying out in pain throughout the incident.

“This was a continued, unlawful assault against a vulnerable 16-year-old boy.”

The court heard how Gittoes had not identified himself as a police officer, explained fully to the youth the reasons why he was being detained, nor did he caution him.

The inspector also left the station before completing a statement and had to be called back by senior colleagues to do one.

Judge Toms said: “No reasonable person in possession of the same facts would have suspected the youth of terrorism.

“This was just a 16-year-old filming for his YouTube channel and was no threat to you or others.

“But you were so riled up by the lack of action taken by your bosses you lost all rational thought.”

Gittoes was released on bail and will be sentenced at Newport Magistrates’ Court on October 27.

IOPC director for Wales, Catrin Evans, said: “While there are occasions when the use of force is required, police officers are entrusted with the power to do so only if it is necessary, reasonable, and proportionate in the circumstances.

“Our investigation examined concerns about the actions of Inspector Gittoes, and the court has found having considered the evidence that the force used in manhandling a 16-year-old boy, who was offering no physical resistance, was excessive and went beyond what was necessary.”