The Independent Office for Police Conduct (IOPC) has confirmed the officer captured on video appearing to punch a suspect several times is under criminal investigation.
The video of the incident in Porthmadog gained significant attention on social media when it emerged last month.
The IOPC said it had “notified a police constable that they are under criminal investigation and have served them a gross misconduct notice”.
They said the development did “not necessarily mean that criminal or disciplinary proceedings will follow”.
North Wales Police have suspended the officer, the IOPC understands.
The force referred the incident to the IOPC to be independently investigated.
The IOPC said at around 11am on 10 May, two officers attended a reported disturbance in the town.
At around 11.50am a man was located by officers in the garden of a nearby house and he was arrested.
The footage extensively shared on social media, shows an officer striking the man to his head a number of times while on the ground.