12-Oct-14: In London, they are monitoring thousands of suspected terrorists

From the MI5 website: current threat level: severe
[Source]

Six weeks ago, we noted here ["29-Aug-14: The British now call risk of a terror attack on the UK 'severe'"] that the UK's Home Secretary had announced her country’s assessment of the terror threat risk level it faces was being raised from "substantial" to "severe". 

The upgrade conveyed the message that an attack was considered "highly likely". Associated Press said that put it at the second highest level (out of five possible categories) on the scale, the highest it had been since July 2011; the highest level on the scale is defined as "imminent".

Since then, there have been several well-publicized arrests of British residents on suspicion of involvement in a significant terror plot. Yesterday's The Guardian ("Police search another London property following arrests of terror suspects") names the latest round of made-in-the-UK suspects as Tarik Hassane, 21, a medical student who studied in Sudan and lives in North Kensington; Rawan Kheder, 20, who lives in Chelsea with his parents, and is described by a neighbour as a “good boy, educated in Britain”; Gusai Abuzeid, a 21 year-old student at the Greenwich School of Management and a part-time worker at a Primark shop who is "really nice and polite [and] would always hold the door for me"; Yasir Mahmoud ("His mum's British... He would come and go but we never knew him to mix with anyone on the estate, we've only seen a couple of lads visit him in the last month or so. They didn't seem to work or do anything, the lad would come in at four or five in the morning, he didn't seem to work..."); and a fifth young man, not yet named.

Now there's additional confirmation this weekend of the robustness of Islamist terrorism and its protagonists via a series of candid disclosures made by London's mayor, Boris Johnson. He says, in a major article in The Telegraph UK on Friday that, contrary to the way the dangers have been presented in various parts of the British and European media

the threat from home-grown terrorist plots was far more widespread than the relatively small numbers of extremists who have gone abroad to fight.

Other key points made by Johnson:

• Until now, it was thought that the main danger came from around 500 jihadis who have travelled to Syria and Iraq from the UK to join Isil or al-Qaeda fighters, around half of whom have returned to Britain. But the Mayor of London suggested the threat from home-grown terrorist plots was far more widespread than the relatively small numbers of extremists who have gone abroad to fight.
• “In London we’re very very vigilant and very very concerned. Every day – as you saw recently, we had to raise the threat level – every day the security services are involved in thousands of operations... There are probably in the low thousands of people that we are monitoring in London... London has a particular concern, because probably of the five or six hundred that are out there, we think a third, maybe more – maybe half – come from the London area. 
• "If and when they come back, we have a real job to deal with them.”

Very, very vigilant sounds like precisely what governments at all levels ought to be, given the very high price of being wrong on this.