Five men and a woman have been arrested on suspicion of being members of the banned far-right National Action party.
Police said the suspects were detained on suspicion of terrorism offences in a planned operation, and that several properties are being searched in connection with the investigation.
The men are a 26-year-old from Cambridge; a 21-year-old from Banbury, Oxfordshire; a 28-year-old from Wolverhampton; a 26-year-old from Leicester; and a 24-year-old from Stockport.
They are all being held in the West Midlands.
A 37-year-old woman, from Banbury, is being held at a station outside the force area.
Several counter-terror units from forces across the country were involved in the operation.
In a statement, West Midlands Police said the suspects were being held "on suspicion of being concerned in the commission, preparation and instigation of acts of terrorism… namely on suspicion of being a member of a proscribed organisation (National Action)."
"A number of properties are being searched in connection with the arrests," the force said.
"The arrests were pre-planned and intelligence-led; there was no threat to the public's safety."
National Action was made a banned group in December 2016, by Home Secretary Amber Rudd.
It is the first extreme right-wing group to be outlawed in the UK. The proscription means that being a member of or inviting support for the organisation is a criminal offence, carrying a sentence of up to 10 years' imprisonment.
In the official list of proscribed groups, it is described as a "racist neo-Nazi group" that was established in 2013 and has branches across the UK which "conduct provocative street demonstrations and stunts aimed at intimidating local communities".
When she banned the group, Ms Rudd said: "National Action is a racist, anti-Semitic and homophobic organisation which stirs up hatred, glorifies violence and promotes a vile ideology, and I will not stand for it.
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"It has absolutely no place in a Britain that works for everyone."
Police last arrested11 people across England and Wales in September 2017on suspicion of being members of the banned group.