Footage has emerged on the internet of the shirtless man apparently being bitten in the neck in the incident in Weston-super-Mare.
Assistant Chief Constable Louisa Rolfe of Avon and Somerset Police said
"The incident has been referred to our professional standards department for thorough investigation as we are keen to understand what happened."
More top news
A couple who lost an unborn baby to complications from twin to twin transfusion syndrome have raised money for research into the condition.
Dorset has a new migrant population - barrel jellyfish are washing up in their dozens on its shores.
The family of one of Britain's oldest poppy-sellers say charities which bombarded her with donation requests were not to blame for her death