A man has been arrested on suspicion of a terror offence with addresses in London and Teesside searched.
The 21-year-old, from the Middlesbrough area, was arrested in north London on Saturday.
Counter Terrorism Policing North East (CTPNE) said he was believed to have been involved in the commission, preparation or instigation of acts of terrorism.
An address in the capital and three on Teesside were targeted by officers.
Bomb disposal experts attended one of the properties, in Teesville, Middlesbrough, to provide advice “following the discovery of potentially suspicious items”.
CTPNE said the arrest was “intelligence-led and pre-planned” and the man was in custody.
Cleveland Police and Counter Terrorism Command are assisting with the investigation.