A 43-year-old man has been arrested on suspicion of terrorism offences.
Counter Terrorism Policing South East detained the man, from Gloucestershire, earlier and he remains in custody.
Officers are searching three locations in the county and another in the West Mercia Police force area as part of their inquiry.
A spokesman said people may see officers in protective suits, but there is believed to be no wider threat to the public at this time.
The man has been arrested on suspicion of encouragement of terrorism contrary to section one of the Terrorism Act 2006; dissemination of terrorist publications contrary to section 2 of the Terrorism Act 2006; and collection of information contrary to section 58 of the Terrorism Act 2000.
He has also been held on suspicion of intentionally encouraging or assisting the commission of serious crime contrary to section 44 of the Serious Crime Act 2007.