The Independent Office for Police Conduct (IOPC) is investigating after a video emerged on social media of a police officer appearing to hit a suspect several times.
A spokesperson for the IOPC said it had decided to launch an independent investigation into the incident in Porthmadog, Gwynedd, after a referral from North Wales Police.
Officers are understood to have arrested a man after being called to reports of a domestic incident on Wednesday.
The IOPC says the man was taken to hospital before he was placed in custody.
A spokesperson for North Wales Police said the force was aware of the footage.
"We are grateful for the contact from members of the public highlighting this incident, but at this time would appreciate further contact being restricted to any potential witnesses to the actual incident, rather than those who have viewed the matter third hand by video or other means," they added.
"As this matter is subject of ongoing investigation the force will not be commenting further regarding this matter at this time."
IOPC director David Ford said: "Footage on social media, capturing part of the interaction between police officers and the arrested man has, understandably, attracted significant interest and public concern.
"It is important that we thoroughly and independently investigate the whole incident, in order to establish whether the level of force used during the arrest was reasonable and proportionate in the circumstances."
Local politicians have written to the chief constable of North Wales Police to express their concern at the footage.
Dwyfor Meirionnydd MP Liz Saville Roberts, Senedd member Mabon ap Gwynfor and local councillor Nia Jeffreys have asked the force what steps it would take "to reassure the public that they can trust the police".