A bomb disposal unit has been brought in as part of a police counter terrorism operation in Sheffield and Chesterfield.
The Army's bomb disposal unit has been deployed to an address in Chesterfield 'as a precautionary measure and in the interest of public safety,' according to the police.
Residents in the immediate area have been evacuated from their homes and the area is cordoned off.
Police officers raided the house and four others in Sheffield this morning in a pre-planned 'intelligence led' operation run by the North East Counter Terrorism Unit and Mi5.
Three men were arrested in Sheffield and one in Chesterfield, with all four being held on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism under section 41 of the Terrorism Act 2000.
The men - aged 22, 31, 36 and 41 - have been taken to a police station in West Yorkshire for questioning.
In Sheffield, two homes in Burngreave, one in Meersbrook and another in Stocksbridge have been searched as part of the operation.
A spokesman for the North East Counter Terrorism Unit, said: "The Army's Bomb Disposal Unit is currently attending the address in Chesterfield. As a precautionary measure and in the interest of public safety, the immediate area has been cordoned off and local residents evacuated while a thorough search is carried out.
"We recognise that local people may have concerns as a result of this activity. We would ask people to remain alert but not alarmed and we are grateful for the assistance and understanding of people locally.
"The public may have heard loud bangs at the time police entered the properties. We would like to reassurance them that this was part of the method of entry to gain access.
"Residents will be kept as informed as possible and we are grateful for their understanding, patience and support while these enquiries continue."
Superintendent Una Jennings, of South Yorkshire Police, said: “I understand our local communities will have concerns about this morning’s police activity but I want to offer my reassurance that we will continue to serve and protect the public of South Yorkshire.
“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.”
Derbyshire Constabulary's Assistant Chief Constable, Bill McWilliam, said: “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.
"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.
“We will be working closely with colleagues at the North East Counter Terrorism Unit as this investigation progresses.”