Police have appealed to the public to help identify "aspiring terrorists" who are showing signs of becoming radicalised.
Mark Rowley, Metropolitan Police Assistant Commissioner, said nearly half of people travelling to Syria were not known as terrorist risks previously.
Rowley said: "Police activity has been escalating for many months now ... Such arrests are being made at a rate fivefold to that of 2013.
"We are appealing to the public, family members and friends to help identify aspiring terrorists; they may be about to travel abroad, have just returned or be showing signs of becoming radicalised."
He called on community and religious leaders to speak out against "warped narratives" to combat extremism.
A British family have told ITV News of their devastation after discovering a close relative is fighting with the Islamic State in Iraq.
A French journalist who was held hostage with James Foley in Syria has told ITV News it was "devastating" to see his murder.