A 38-year-old man has been arrested on suspicion of explosives offences in Bradford following an investigation by Counter Terrorism Policing North East (CTUNE).
A CTUNE spokeswoman said he has been taken to a local police station for questioning.
She said three residential addresses in Bradford are being searched in connection with the investigation and "the arrest was pre-planned and intelligence led".
The spokeswoman said: "Following the discovery of some items at one of the properties in Bradford, explosives ordnance disposal have been called to provide specialist advice.
"A number of local residents have been evacuated as a precautionary measure while examinations are ongoing."