A 38-year-old man has been arrested on suspicion of explosives offences as a bomb squad joined police searching homes in Bradford.
Counter Terrorism Policing North East (CTPNE) said three residential addresses in the city were being searched on Friday.
A spokeswoman said: "A man from Bradford has been arrested as part of investigation by Counter Terrorism Policing North East earlier today.
"The 38-year-old man was arrested on suspicion of explosives offences and has been taken to a local police station for questioning.
"The arrest was pre-planned and intelligence-led.
"Following the discovery of some items at one of the properties in Bradford,
EOD (Explosive 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."