A man has been arrested on suspicion of being a member of banned far-right group National Action.
The 46-year-old was held at a property in Lincoln on suspicion of being a member of a proscribed organisation and malicious communications.
The arrest is linked to an investigation by the North West Counter Terrorism unit.
Police said the man has been released under investigation pending further inquiries.
Det Ch Supt Dominic Scally, head of counter terror policing for the North West, said the arrest "forms part of a wider operation into the activities of a proscribed organisation".
"Those involved in proscribed organisations have a detrimental effect on the communities others work so hard to build, so it is essential people contact us with information so we can put a stop to those who support extremist ideologies."
Lincolnshire Police, which supported counter terrorism officers in the arrest, said it was a pre-planned operation and it is "not believed there is any threat to the local Lincolnshire community at this time".
National Action was banned in December 2016.