Britain is currently running more than 500 terror investigations, with its intelligence services operating on a bigger scale due to the increase in threats, the head of MI5 said Tuesday.
"The scale at which we are operating is greater than ever before," said Andrew Parker, head of Britain's domestic intelligence agency, in a speech in London.
"We are now running well over 500 live operations involving around 3,000 individuals known to be currently involved in extremist activity in some way."
British security forces are confronted by an increasing number of attack plots and have managed to foil 20 over the past four years, the MI5 chief said.
"Just in the last seven months, we and the police have thwarted seven terrorist plots by Islamist extremists" of the total 20.
Britain has been struck by five terror attacks so far this year, four in London and one in Manchester. They included bombings and deadly violence with vehicles and knives.
Some of the assailants were among the 20,000 people known to security services.
Parker said the threat is "more diverse than I've ever known" and involves "extremists of all ages, genders and backgrounds".
And he warned the plots could be carried out very quickly.
"Attacks can sometimes accelerate from inception through planning to action in just a handful of days," he said.
"This pace, together with the way extremists can exploit safe spaces online can make threats harder to detect and give us a smaller window to intervene."
Plots are developed both within the UK, abroad and online, he said, warning of the ease with which potential attackers can learn how to build a bomb on the internet.
"We can't hope to stop everything," he admitted, despite a record 379 terror-related arrests in the 12 months to June.
Four assailants were killed by police this year during the course of attacks in London: one outside parliament and three others near London Bridge.
Another extremist died in a suicide bombing in Manchester.
Two other men have been charged over separate terror attacks -- a bomb which partially detonated on the London Underground last month, and an attack on worshippers outside a mosque in the capital in June.