Terror-related arrests in England, Wales and Scotland reached record levels last year, when 338 people were held, Scotland Yard has said.
More than half of those arrests were related to Syria, Met Police assistant commissioner Mark Rowley said.
Police said more than 700 potential terror suspects are now thought to have travelled to Syria.
About half of those people – said to be of “significant concern” – are believed to have since returned to the UK.
The total number of terror-related arrests was up from 254 in 2013/14 – an increase of about a third.
Of the latest arrests, 56 related to suspects under the age 20 – a figure that police say represents “an emerging trend”.