Six people have been arrested on suspicion of terrorism offences in a series of morning raids across the country, police said.
Four men from Derby aged 22, 27, 35 and 36, a 27-year-old man from Burton upon Trent and a 32-year-old woman from London have all been arrested on suspicion of engaging in the preparation of an act of terrorism. They remain in police custody.
Six properties in Derby, Burton and London are being searched as part of the three force-strong investigation.
It is being led by the North East counter terrorism unit supported by local officers from Derbyshire, Staffordshire and the Metropolitan Police. Local officers will be patrolling the areas to speak to anyone with concerns, police said.
Forensic teams have been seen entering a property in Leopold Street in Derby, while the road has been closed to traffic.
Steven Allen, 32, who lives in Leopold Street, said: "I've just come out of my flat, saw a police car there, thought it a bit odd.
"I heard absolutely nothing, I literally got out my flat door, saw a police car, saw the vans. It's usually a very quiet street.
"It's just such a shock to the system really because this is a very quiet street."