Six people have been arrested on suspicion of offences under the Terrorism Act.
Four men from Derby, a man from Burton upon Trent and a woman from London were arrested on suspicion of engaging in the preparation of an act of terrorism.
Six properties in Derby, Burton upon Trent and London are all being searched as part of the operation.
Counter-terrorism sources described the operation as “significant” and the result of an ongoing investigation.
The sources said the investigation was into a group believed to be working towards an attack – though a strike was not thought to be imminent.
The four suspects from Derby are aged 22, 27, 35 and 36. The suspect from Burton upon Trent is aged 27 and the woman from London is 32.
The investigation is being led by the North East Counter Terrorism Unit, supported by local officers from Derbyshire, Staffordshire and the Metropolitan Police.
There is a large police presence in Leopold Street in Derby but police have not confirmed if it is part of the investigation.
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.”
Adam Mehmood, who owns Happy Footwear nearby, said: “It’s sad and scary. It’s not good for the community and the people who live round here.
“When I came to work I heard the news. It’s a big shock.”
Local officers will be patrolling the areas to speak to anyone with concerns, police said. BBC News