British counter-terror police resumed searches of two properties on Wednesday following the arrest of four men on suspicion of planning acts of terrorism on Tuesday.
The men have been detained on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism under Section 41 of the Terrorism Act 2000.
Police raided five properties in South Yorkshire and Derbyshire in England.
A 31-year-old man was arrested after the bomb squad cordoned off a street in Chesterfield and nearby residents were evacuated from their homes as searches were carried out.
Three other men aged 22, 36 and 41 were detained in separate properties in the Burngreave and Meersbrook areas of Sheffield.
An additional address in Stocksbridge in Sheffield was also searched in connection with the arrests.
The four suspects were taken to a police station in West Yorkshire for questioning and remain in custody.
Police said the arrests were "intelligence-led and preplanned" as part of an on-going joint operation by Counter Terrorism Policing North East and Britain's domestic security agency MI5.
Una Jennings, South Yorkshire Police Superintendent for the Sheffield area, said: "We work very closely with our colleagues at the North East Counter Terrorism Unit and we will ensure we maintain these strong links in order to provide a high level of service and delivery to everyone in our community. Public safety is our utmost concern."
Police added in a statement, "the public many have heard loud bangs at the time police entered the properties. We would like to reassure them that this was part of the method of entry to gain access".
"We of course understand that police activity of this nature can be unsettling. However, please be reassured: the arrest we wanted to make has been made," Bill McWilliam, Derbyshire Constabulary assistant chief constable said.
"Our advice remains to be vigilant, which is not different to our day-to-day advice in the current climate, but continue to go about your business as usual."
The UK threat level for terrorism remains at severe, meaning an attack is deemed highly likely.
Britain has been hit with five attacks this year, in London and Manchester, and UK security officials said earlier this month that authorities have foiled nine terror plots since March.