British counter-terrorism forces are monitoring "thousands" of potential extremists in London, Mayor Boris Johnson said yesterday, as police continue to question five men arrested on suspicion of terrorist offences.
His comments came as police chiefs warned officers throughout Britain to use extra care because of an increased threat to their personal safety
Mr Johnson said the security forces in London are involved in "thousands of operations" every day, adding that the number of people being watched is "in the low thousands".
"In London we're very, very vigilant and very, very concerned," he said, adding that the national threat level had recently been raised.
It is now listed as "severe", meaning an attack is viewed as highly likely.
He said many of the estimated 500 fighters who had gone from Britain to Syria to support Islamic State militants had come from the greater London area, posing a security threat if they return after having received training in explosives and weapons use.
Assistant Metropolitan Police Commissioner Mark Rowley said the new warning to police officers about their safety was made because "the threat level to police officers and staff has been raised".
He did not specify whether the increased threat to police is related to the arrests of the five men earlier this week.