Police have confirmed that a 21-year-old suspect has been arrested under the Terrorism Act after four properties were searched on Saturday night.
The man was held in north London in a pre-planned operation led by Counter Terrorism North East and Cleveland Police.
One address on North London, two in Middlesbrough and one in Teesville, near Middlesbrough, have been searched.
The search at one Middlesbrough address and the Teesville property are continuing.
The man is in custody after being held on suspicion of being involved in the commission, preparation or instigation of acts of terrorism contrary to Section 41 of the Terrorism Act 2000.
Chief Superintendent Dave Sutherland of Cleveland Police said: "I know that the nature of this police activity will be concerning to residents in our local communities, and I want to reassure people that safety is our main priority.
"You will see increased patrols from the neighbourhood teams and I would ask anyone with concerns to speak to these officers.
"The activity is intelligence-led and pre-planned and we are working closely with our colleagues from Counter Terrorism Policing North East.
"Our aim is to keep people as updated and informed as we are able to given the sensitive nature of this investigation.
"We appreciate the public's patience and understanding while we carry out this work."