There are at least 2,000 people in the UK who pose a threat to national security because of their support for terrorism, the head of MI5 has said.
Jonathan Evans took over as director of MI5 in April
Jonathan Evans said there had been a rise of 400 since November 2006.
He said children as young as 15 were being recruited for terrorist-related activity by al-Qaeda.
Resources that could be devoted to counter-terrorism were instead being used to protect the UK against spying by Russia, China and others, he added.
There had been "no decrease" in the number of Russian covert intelligence officers operating in the UK since the end of the Cold War, Mr Evans said in a speech in Manchester.
"A number of countries continue to devote considerable time and energy trying to steal our sensitive technology on civilian and military projects, and trying to obtain political and economic intelligence at our expense."
It was "a matter of some disappointment", he said, that this ongoing threat continued to take up significant amounts of equipment, money and staff.
Mr Evans took over as director general of MI5 in April from Dame Eliza Manningham-Buller.
Speaking on Monday at the Society of Editors' annual conference, he said the number of individuals in the UK causing concern had risen in part due to better intelligence gathering in "extremist communities".
"But it is also because there remains a steady flow of new recruits to the extremist cause."
In order to gather recruits, Mr Evans said, extremists were methodically and intentionally targeting vulnerable young people and children.
The UK had to do more to protect these young people, he added.
Mr Evans said attacks on the UK were "not simply random plots by disparate and fragmented groups", but part of a "deliberate campaign" by al-Qaeda.
In the past 12 months, MI5 had found links between an increasing range of countries and terror plots in the UK, he said.
In Iraq, Algeria and parts of East Africa, especially Somalia, he said, the "al-Qaeda brand" had expanded and now posed a threat to the UK.
Mr Evans said he did not think the level of terror threat against the UK had "reached its peak".
"We will do our utmost to hold back the physical threat of attacks, but alone, this is merely containment.
"Long-term resolution requires identifying and addressing the root causes of the problem."
He said it was "inevitable" there would be individuals who came to police or security service attention, but were still able to go on to carry out acts of terrorism.
"Every decision by the security service to investigate someone entails a decision not to investigate someone else. Knowing of somebody is not the same as knowing all about somebody."
BBC security correspondent Frank Gardner said the speech contained the message that MI5 needed the public's help.
"It's about tackling the ideology at grass roots. They can only really tackle the symptoms. They can't go up to people and say, 'Do you follow al-Qaeda?'"
Shiraz Maher, a former member of radical Muslim group Hizb ut-Tahrir, said the recruitment of young people by militant groups was a reality.
Youth initiatives, including football training and anti-drugs schemes, were being used to groom "impressionable and idealistic" young people, he told BBC Radio 4's The World at One programme.
But the Ramadhan Foundation said it was concerned Mr Evans had not stressed the 2,000 people suspected of involvement in extremist activity made up only a small proportion of the 1.6 million Muslim population.
Mohammed Shafiq, a spokesman for the Muslim youth organisation, said the language was inflammatory and called for responsible dialogue.
He said the group was prepared to talk to the police and security services, but in order to defeat terrorism it was important to acknowledge the threat existed mainly due to foreign policy.
Liberal Democrat shadow home secretary Nick Clegg highlighted the timing of the release of these figures which comes a day before Gordon Brown's announcement on new anti-terror powers.
"It is crucial that the security services do not get drawn into politics," he said.
During his speech, Mr Evans also announced that MI5's new Northern Ireland headquarters would soon be formally opened and said that by 2011 25% of the service's staff would be based outside London.