London ? British authorities are tracking almost 30 terrorist plots involving 1,600 people, the country’s domestic spy chief said in remarks released Friday, warning that young British Muslims are at risk of being radicalized by extremists.

It was the first public estimate of the threat by the head of MI5, Dame Eliza Manningham-Buller, who said her agency and police are monitoring 200 cells actively engaged in plotting or aiding attacks in Britain and abroad.

More than a year after the suicide attacks on London’s transit system, Manningham-Buller predicted the fight against terrorism would last a generation.

“It is a sustained campaign, not a series of isolated incidents,” she said in a speech to a small audience of academics Thursday, according to a transcript released by the spy agency on Friday. “Radicalizing elements within communities are trying to exploit grievances for terrorist purposes.”

“It is the youth who are being actively targeted, groomed, radicalized and set on a path that frighteningly quickly could end in their involvement in mass murder of their fellow U.K. citizens, or their early death in a suicide attack or on a foreign battlefield,” she said.

Manningham-Buller said MI5 had foiled five major plots since the July 2005 transit bomb attacks in London, which killed 52 commuters and the four suicide bombers.

Officials are also “aware of numerous plots to kill people and to damage our economy,” she said. “What do I mean by numerous? Five? Ten? No, nearer 30 that we currently know of.”

Prime Minister Tony Blair said he agreed with the spy chief’s assessment that the threat from international terrorism “is serious, is growing, and will, I believe, be with us for a generation.”

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 led 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.