Andrew Parker highlighted the "short life" of some attack plans as he described how the danger to the UK is now at the highest tempo seen at any point in his 34-year intelligence career.
In rare public comments, he also set out how extremists can exploit "safe spaces" online, making the task of disrupting their activities more difficult.
And he issued a fresh challenge to technology firms, saying they have an "ethical responsibility" to help governments confront the threat.
In his first substantial remarks since Britain was hit by a flurry of attacks this year, Mr Parker detailed the "acute and enduring" challenge posed by Islamist terrorism.
Twenty plots have been foiled over the past four years, including seven in the last seven months.
"We've seen a dramatic up-shift in threat this year," Mr Parker said in a speech in central London.
"Today there is more terrorist activity, it's coming at us more quickly and it can be harder to detect."
Security services and police are operating at a scale which is "greater than ever before", with well over 500 live operations involving around 3,000 individuals.
Agencies must also assess the risk posed by those returning from Syria and Iraq, and the "growing pool" of more than 20,000 people looked at as part of previous terror investigations.
Mr Parker also noted that there will be some violent extremists "not yet known to us at all".
He said the shift is being driven by the "murderous strategy" of Daesh (also known as Islamic State) and online propaganda, and warned that the threat is "more diverse than I've ever known".
"Plots developed here in the UK but plots directed from overseas as well," he said. "Plots online. Complex scheming and also crude stabbings. Lengthy planning but also spontaneous attacks.
"Extremists of all ages, gender and backgrounds united only by the toxic ideology of violent hatred that drives them.
"These threats are sometimes now coming at us more quickly - whether crude but lethal attack methods, for example using a knife or a vehicle, or more sophisticated plots, when in today's world terrorists can learn all that they need online to make explosives and build a bomb."
He said attacks can sometimes accelerate "from inception through planning to action in just a handful of days", adding: "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."
Among the plots seen in recent years some have had a "very short life from first inception to intent to act", Mr Parker said.
"Some of them would be measured in days, at the extreme," he added.
While Daesh is "rapidly losing ground" in its heartlands in Syria and Iraq, tackling it as a movement will require "sustained international focus for years to come", the Director General of MI5 said.
He made clear that counter-terror agencies "can't hope to stop everything", acknowledging that "attacks will occur sometimes".
But he said the vast majority are being found and stopped and a "perfect standard of 100%" is not achievable.
Striking a defiant tone, he described the response to the threat as "unrelenting".
Mr Parker said: "Those that wish our country harm can expect to meet MI5 and the police. They will face the full force of the law and be brought to justice."