Police in the UK are keeping watch on "thousands of people" who may be involved in terrorism, Scotland Yard's head of counter-terrorism says.
Peter Clarke has overseen the airline bomb plot inquiry
Peter Clarke told a BBC Two documentary Al-Qaeda: Time To Talk? that his officers had to be focused on a "whole range of people".
"Not just terrorists not just attackers but the people who might be tempted to support or encourage," he said.
He recently described the intelligence picture in the UK as "very disturbing".
BBC reporter Peter Taylor, who spent a year investigating the terror threat, asked Mr Clarke roughly how many people the police were looking at believing they could be involved directly or indirectly in terrorism.
Mr Clarke said: "I don't want to go down the numbers game, I don't think it's helpful.
"All I can say is that our knowledge is increasing and certainly in terms of broad description, the numbers of people who we have to be interested in are into the thousands."
Since the terror attacks on the US on 11 September 2001, Mr Clarke said the police had learnt that the threat was not something that came just from overseas.
"What we've learnt, and what we've seen all too graphically and all too murderously, is that we have a threat which is being generated here within the United Kingdom."
Mr Clarke confirmed there was a form of "pipeline" that takes young Muslims from the UK into Iraq.
"What we do see is individuals with connections who are happy to try to organise the travel of others," he said.
The most senior British officer in Iraq, Lt Gen Sir Rob Fry, said he had seen no evidence of young British men travelling to Iraq to be involved in terrorism.
He said: "Has Iraq contributed to the radicalisation of Islamic youth? I honestly don't know - but it doesn't need Iraq to do that. There are enough examples of similar situations in the world. It could be Palestine, it could be Kashmir, it could be the longer term emasculation of Islamic societies.
"I think that there is enough general alienation and disillusionment within the Islamic belief system, both within the Middle East, and also in places like Great Britain for that to happen, without any specifics being drawn back to Iraq."
Previous estimates had suggested a sharp growth in the numbers of people within the UK who were prepared to involve themselves in terrorism.
A report by the Intelligence and Security Committee into last year's 7 July London bombings claimed that MI5 was aware of about 250 possible suspects in 2001.
By July 2004, the number of "primary investigative targets" the service was aware of had doubled to 500, according to the MPs' report.
A year later the number had leapt again to around 800, the report said.
In the lead-up to the first anniversary of the London attacks, Mr Clarke said Scotland Yard was running more counter-terrorism investigations than ever before.
Al-Qaeda: Time to talk? will be shown on Sunday 3 September on BBC Two at 2100 BST.