He warned it could take years to defeat the Islamic terrorism that claimed 52 lives in the July 7 attacks and said the cost of the police response to the threat had already reached £60m.
Mr Clarke told MPs that details of new anti-terrorism laws would be published this week, and announced he had, for the first time, imposed a "control order" on a British national. He spelt out the effects of the bombings in a special hearing of the Commons Home Affairs Select Committee on the July bombings.
Amid fears of homegrown terror cells, he said: "There are certainly hundreds of people who we believe need to be very closely surveilled because of the threat they offer."
Mr Clarke also said there was increasing evidence the suicide attackers had foreign contacts. He said the taped message left by the suicide bomberMohammad Sidique Khan, was being analysed for clues over where it was produced and by how it was distributed.
He announced that he had imposed a control order under the Prevention of Terrorism Act on a British citizen, restricting his movements. He is understood to be a foreign-born man who has been naturalised.
He refused to be drawn on a timescale for the terror threat when asked whether it could last for 30 years, as had the IRA bombing campaign, but he warned that it could be a "considerable time". He said: "The fact is that we have what I would call a nihilist terrorist threat, something that will only be beaten by demonstrating it cannot succeed."
The Home Secretary told the committee that the cost to the Metropolitan Police of coping with the aftermath of the July bombings had reached £60m by 1 September, half representing overtime payments. "It's the obligation of the Government to try and deal with that," he said. Mr Clarke defended the decision of the security services to downgrade the level of alert shortly before the bomb attacks on July 7. He said they had no prior knowledge of the atrocities and had foiled two terror plots last year.
Challenged on the use of stop and search against Muslims and other Asians, Sir Ian Blair, the Metropolitan Police Commissioner, said: "We are not in the business of stopping and searching people who fit a particular profile... But on the other hand, because all the bombers were men, saying we are not going to search women would be foolish."
Andy Hayman, the Met'sAssistant Commissioner of Specialist Operations, disclosed the investigation had gathered 38,000 pieces of evidence from 160 crime scenes, filling two warehouses. Eighty thousand videos had been seized some of which had to be viewed more than once and 1,400 fingerprints taken.
Ken Livingstone, the Mayor of London, said technology to detect explosives on the Tube was being investigated by transport chiefs. He said there would be 100 per cent CCTV coverage on London's buses by the end of the year. The number of cameras on the Tube would be doubled over the next five years.
The Mayor said serious problems remained with police radio systems on the Underground, which he blamed on the Government's partial sell-off of the system under the private finance initiative.
* Ken Livingstone provoked controversy yesterday when he compared the Muslim cleric Yusuf al-Qaradawi, who has been banned from the US after praising suicide bombers, with John XXII, the reforming pope from 1958 to 1963. He said the men were "very similar". He described the cleric as a "powerful progressive force for change".Reuse content