LONDON -- It's a nightmare scenario that police and the public don't want to think about but can't afford to ignore: What if the terrorists behind last week's deadly bombings strike again?
Authorities have warned that the terror cell that carried out Thursday's bombings of three crowded rush-hour Underground trains and a double-decker bus may be intact and capable of another strike.
"Our fear is of course of more attacks," Home Secretary Charles Clarke, the Cabinet minister responsible for law and order, said Sunday.
It didn't happen after al-Qaida's Sept. 11, 2001 attacks in New York and Washington, or after the March 2004 train bombings in Madrid, Spain.
But authorities, warning that anything is possible, said they're working to contain the threat by boosting police patrols, deploying more undercover officers and restricting the movements of known suspects. Some London hotels have been using electronic wands to search guests for weapons or explosives.
Lawmakers, meanwhile, are renewing a push to introduce a controversial national system of high-tech biometric ID cards.
The measures being taken or considered suggest Britain is following the lead of the United States, Israel, Russia and other countries that have responded to attacks with vows to toughen security - often with mixed success and criticism from citizens wary of greater government and police powers.
Conservative leader Michael Howard called anew Sunday for extra security measures, including the appointment of a minister of homeland security - a step Washington took after Sept. 11.
"Obviously we must remain prepared for any eventuality. The fact that we've had these attacks doesn't mean we won't have more attacks," said Andy Trotter, deputy chief constable of the British Transport Police.
"Therefore, we're taking all necessary precautions to keep London as safe as we can," he said. "You'll see the activities out there on the street: the high visibility policing. The undercover work you won't see, of course. At the same time we are appealing to Londoners to assist us ... by reporting anything suspicious."
Although authorities have not ruled out the possibility that the terrorists were British rather than foreigners, Clarke said the government was tightening border security through an "e-borders" system that subjects people to computer checks as they enter and leave Britain.
Yet there were few signs of a greater police presence on the streets of London, where the prevailing mood was a sense that not even a lockdown would eliminate the threat of more attacks.
"Cities are made up of millions of soft targets. They are an impossible security problem," said Steve Graham, a terrorism expert. "On 9/11, it was the air system. In Madrid, it was the rail system. In London, it was the Tube and bus system."
Prime Minister Tony Blair said his government was operating on the theory that "you have got, as a government, to do everything you can to protect your people."
"But if people are actually prepared to go on to a Tube or a bus and blow up wholly innocent people, people just at random ... you can have all the surveillance in the world and you couldn't stop that happening," he added.
Even before the attacks, Blair had been tightening security laws.
In March, parliament passed the Prevention of Terrorism Act, which allows authorities to place suspected terrorists under house arrest and impose travel bans without trials. But the government's push for national ID cards including fingerprints and iris scans has met with resistance from civil libertarians.
Blair, alluding to that opposition and the need to balance security concerns against human rights and privacy, conceded his government has "got to be very cautious about it." An ID card system, he said, would have to be "hedged around an enormous amount of restrictions on government power."
Israelis are accustomed to heavy security; Londoners are not.