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."
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Chillier this evening with largely clear skies.
Mainly dry with sunny spells this afternoon.
A thief who tried to snatch £20 from a disabled teenage girl has been jailed after she fought him off and passers-by pinned him down.