Police have arrested two men on suspicion of planning acts of terror.
Loud bangs were heard as Counter Terror officers forced their way into two addresses in Savile Town, Dewsbury, West Yorkshire.
Counter Terrorism Policing North East said men aged 52 and 21 were arrested "on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism".
They have been detained at a police station in West Yorkshire for questioning, and two homes were being searched in connection with the ongoing investigation.
A spokeswoman for the unit said: "The public may 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."
The arrests were planned and "part of a proactive, intelligence-led investigation", the unit said.
They were not linked to an ongoing malicious communications investigation into the so-called "Punish a Muslim" letters.
West Yorkshire Police Superintendent Marianne Huison 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 West Yorkshire.
"We work very closely with our colleagues in CT Policing North East and with local partners 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."