A wildlife campaigner said he was held for 10 hours after saying he drowned a grey squirrel to provoke the RSPCA into taking him to court.
Norris Atthey, from Northumberland, said he killed the animal to confront the charity for prosecuting another man last month for doing the same thing.
The 66-year-old is chairman of Morpeth Red Squirrels, which works to protect native red squirrels in the area.
The RSPCA said its investigations were "ongoing".
Mr Atthey, of Ulgham, said he believed drowning was a humane way of killing squirrels.
Mr Atthey said he was angry about the prosecution of the other man last month who had admitted causing unnecessary suffering to an animal by drowning a grey squirrel in a water butt.
An RSPCA inspector visited Mr Atthey on Wednesday and said that when he would not co-operate, police were called.
Mr Atthey said he was taken to a police station at 1100 BST and allowed home about 2100 BST.
He admitted part of the reason he was detained so long was that he argued he was being held unlawfully.
Mr Atthey said: "I am incensed about the whole affair.
"People cannot believe what's going on. My solicitor was gobsmacked."
Northumbria Police said: "Police were called to assist RSPCA officers at an address in Ulgham at around 10.45am yesterday.
"Officers arrested a 66-year-old man at the address for causing unnecessary suffering to an animal."
If the charity decides to prosecute, Mr Atthey will be summoned to appear before magistrates.
An RSPCA spokesman said police had been called when Mr Atthey refused to talk to an inspector.
He said: "Investigations are ongoing and we are unable to make any further comment at this stage."
Grey squirrels have been blamed for the decline of red squirrels in Britain.