Five men and a woman have been arrested on suspicion of belonging to banned far-right terror group National Action.
West Midlands Police said the suspects, aged between 21 and 37, were detained under Section 41 of the Terrorism Act on Wednesday.
The arrests were pre-planned and intelligence-led with no threat to the public's safety, the force said.
National Action was made a proscribed terrorist organisation in December 2016, the first extreme right-wing group to be outlawed in the UK.
The male suspects include 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.
The female suspect is a 37-year-old, also from Banbury, West Midlands Police said.
All six are being held at a police station in the West Midlands.
Officers are now carrying out searches at several properties in connection with the arrests.
A string of counter-terrorism units were involved in the operation, including from the West Midlands, North West, South East and East England.
They are the latest in a wave of arrests of alleged National Action members since the group was banned by the Government.
Counter-terror police have detained dozens of people, including some on suspicion of plotting terror attacks, since the order formally came into effect.