Police are dealing with “exceptionally high” numbers of terrorist investigations and foil several murderous plots a year, terrorist experts have said.
Scotland Yard Assistant Commissioner, Marc Rowley, who is the National Policing Lead for Counter Terrorism, took the unusual step of revealing details of the scale of the deadly threat that police face.
It came as Foreign Secretary Philip Hammond suggested British jihadists who go to fight in Iraq or Syria could be tried for treason.
Mr Rowley said: "So far this year we have made 218 arrests and are running exceptionally high numbers of counter-terrorism investigations, the likes of which we have not seen for several years. Against an increasing operational tempo we are disrupting several attack plots a year.
"These plots are of varied sophistication, from individuals planning to carry out spontaneous yet deadly attacks to more complex conspiracies, almost all seemingly are either directed by or inspired by terrorism overseas. We have secured evidence and charged 16 returnees for terrorist activity in Syria.
"The volume, range and pace of counter-terrorism activity has undergone a step change. Public safety is our number one priority and we will always focus our disruption activity against those posing the greatest and most imminent threat. Sometimes this means intervening very early - essential to prevent attacks, but presenting enormous challenges in securing sufficient evidence to charge."
More than 80 per cent of the material removed from the internet is related to Iraq and Syria, and Mr Rowley revealed that so far there are 66 people who have been reported missing to police who are feared to have travelled to Syria.
He added: "The growing problem of young, impressionable, and in some cases vulnerable, individuals being radicalised online is an increasing risk. Extremist groups are using social media in highly sophisticated ways in order to recruit or persuade individuals towards their violent and warped ideologies.
"Dozens of families have contacted us concerned about loved ones or information when they believe someone is travelling abroad. It is only through courageous action like that that we can help and we have been supporting such families in every way we can."
The senior officer's comments came after Scotland Yard made a series of arrests over an alleged "significant" plot cooked up by suspects including at least one man who was believed to have travelled to Syria.
Another series of swoops by counter-terrorism teams in the South East saw three men and three women arrested following raids at four addresses in Portsmouth and Farnborough in Hampshire, and Greenwich, south east London.
There were also much publicised appeals to try to find missing teenagers Samya Dirie, 17, and Yusra Hussien, 15, who vanished last month and are thought to have travelled to the war-torn state.
Meanwhile, Mr Hammond said Britons travelling to the Middle East had "sworn personal allegiance" to Islamic State (IS) and could potentially have committed treason.
The last prosecution for treason was in 1946, when the notorious Lord Haw Haw - real name William Joyce - was hanged for his Nazi propaganda broadcasts.
The offence remains on the statute books, even though the death penalty has been abolished.
Additional reporting by the Press Association