The UK is facing a "multidimensional threat" that is rapidly evolving "at a scale and pace we've not seen before", the director general of MI5 has warned.
Andrew Parker said the security service is operating at an unprecedented level and has seen "a dramatic upshift in threats this year".
"It's at the highest tempo I've seen in my 34-year career," he said. "Today there is more terrorist activity, coming at us more quickly, and it can be harder to detect.
"We're now running well over 500 live operations involving around 3,000 individuals known to be currently involved in extremist activity in some way.
"As well as those we are looking at today, risk can also come from returnees from Syria and Iraq and also the growing pool of over 20,000 individuals we've looked at in the past in our terrorism investigations.
"And there will be some violent extremists not yet known to us at all."
Speaking in central London during a rare public appearance, Mr Parker paid tribute to MI5's 4,000 intelligence officers and the agents who risk their lives operating undercover to protect the country.
"They get up and come to work every single day to make terrorist attacks less likely and to keep the country safe," he said.
"They're constantly making tough professional judgements based on fragments of intelligence, pinpricks of light against a dark and shifting canvas.
"That is the job of MI5. When an attack happens, everyone in MI5 is deeply affected, on a personal and professional level."
Separately, Mr Parker sat down for television interviews with the UK's three major broadcasters - the first time any serving British intelligence chief has done so.
He told Sky News that tech companies have an "ethical responsibility" to help counter terrorism, adding that he wanted to create partnerships with them.
"All those (technological) developments that we have give opportunities at the edges to terrorists as well," he said.
"I don't believe that any of these companies want those unintended side effects. They don't want to be helping terrorists acquire the materials that they need for online purchasing.
"They don't want to be helping them in encrypted communication avoid detection by MI5 and they don't want to have their social media platforms used for terrorist propaganda."
Mr Parker added that "some helpful action is being taken", but said there is "a challenge of pace, volume and reach as these technologies continue to develop so rapidly."
For the first time, the MI5 director general also revealed the existence of a European joint operational centre.
The facility is based in the Netherlands and brings together intelligence officers from Europe's major agencies.
Mr Parker said the operational centre had prevented attacks and captured more than a dozen terrorists "who might not otherwise have been found in time".
He added: "We don't just stand with our European colleagues, we work with them. We share intelligence. We run joint operations every single day.
"Only last week I met again with my counterparts from 30 European security services, known together as the Counter Terrorism Group or CTG, as we decided on the next stages of collective action."
The joint intelligence centre operates separately to the EU, but the fact that Mr Parker dedicated a significant chunk of his speech to address European co-operation was no accident.
Brexit creates uncertainty for the intelligence agencies, as it does many British organisations.
Mr Parker was honest, he said the threat to the UK is evolving at a scale and pace not seen before.
It is a huge challenge to MI5 - a generational threat and there is no end in sight.