Labour councillors and local election candidates have been warned about their personal safety after death threats were issued in an anonymous letter to a police chief.
The highly politicised letter concerning named members of Durham County Council was sent to Durham Chief Constable Mike Barton.
The force is investigating and said it has provided reassurance to those identified in the letter.
Councillor Carl Marshall, chairman of County Durham Labour, said the letter is "a threat to the democratic process".
He said: "Abusive correspondence is something that is sadly becoming increasingly common, but threats to the lives of Labour councillors is very concerning and not something that we have ever had to deal with.
"We have issued police guidance to all Labour councillors and prospective Labour candidates to be more aware of their own personal security and also to help in any way with the ongoing investigation.
"It would be easy to dismiss the threats contained in the letter, but the tragic murder of Labour MP Jo Cox serves to prove that there is a real threat facing those who work in politics, whatever party they represent."
A Durham Police spokeswoman said the letter addressed to the chief constable was received on Thursday, March 2.
"The letter contained a great deal of political rhetoric, along with threats to a large number of county councillors," she said.
"There is no supporting intelligence to corroborate these threats.
"Unfortunately, letters of this nature are not uncommon and are generally sent to cause concern to the recipients without any real substance behind them.
"However, inquiries are taking place to trace the person responsible for the letter and we are liaising with Durham County Council to offer reassurance to those named."