"It's a very long and deep struggle but we have to stand up and be counted for what we believe in and take the fight to those people who want to entice young people into something wicked and violent but utterly futile," Blair told journalists at his 10 Downing St. office.

Manningham-Buller said the threat includes some networks directed by al-Qaida in Pakistan and others more loosely inspired by the group, and that their plans include mass-casualty suicide attacks in Britain.

The plots "often have linked back to al-Qaida in Pakistan, and through those links al-Qaida gives guidance and training to its largely British foot soldiers here on an extensive and growing scale," she said.

Manningham-Buller, who has headed MI5 since 2002, warned that radicalization, especially of young people, was one of the biggest problems facing anti-terror investigators. Three of the four men who attacked three London subways and a red double-decker bus on July 7, 2005, were British-born.

"More and more people are moving from passive sympathy towards active terrorism through being radicalized or indoctrinated by friends, families, in organized training events here and overseas, by images on television, through chat rooms and Web sites on the internet," she said.

In August, police said they had foiled a plot by a British terrorist cell to blow up trans-Atlantic airliners. More than a dozen people, all British, are awaiting trial in the case.

On Tuesday, a British Muslim convert, Dhiren Barot, was sentenced to life in prison for plotting to attack US financial landmarks and blow up London targets with limousines packed with gas tanks, napalm and nails.

Inayat Bunglawala, spokesman for the Muslim Council of Britain, said Manningham-Buller had given "a very sobering warning." He said, however, it was essential that "British Muslims are seen as a partner in the fight against terrorism and not some sort of community in need of mass medication."

"Holding a community responsible for the actions of a few would be counterproductive," Bunglawala said.

Bill Durodie, senior lecturer in risk and security at the UK Defense Academy, said high-profile speeches and headline-grabbing numbers risked exaggerating the scale of the threat facing Britain.

"It's easy to pull out alarmist headlines," he said. "What we're seeing here on the whole are lone individuals (and) small groups."

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