Police have arrested six people in anti-terrorism raids across central England and London, with bomb disposal officers later dispatched as officers carried out searches.
Detectives detained four men from Derby and another in Burton upon Trent in central England while a woman was also arrested in London on suspicion of preparing an act of terrorism.
Police said the arrests were linked to "international related terrorism".
"We would ask people to remain alert and vigilant but not to be alarmed," a statement from the North East Counter Terrorism Unit said.
Six properties were being searched as part of the investigation and an Army bomb disposal team had been sent as a precaution to the Normanton area of Derby. No houses were evacuated, police said.
The five men, aged 22, 35, 36, and two 27-year-olds, and the 32-year-old woman, were in police custody where detectives can question them for 48 hours before charging, releasing them or applying for permission to detain them for longer.
Britain is on its second highest threat level, meaning an attack by militants is considered highly likely. Last week Britain's foreign intelligence chief said Islamic State militants were using turmoil in Syria to plot attacks against the United Kingdom and her allies.