LONDON — With other suspects in an alleged terrorist plot to blow up U.S.-bound jetliners still at large late Thursday, officials weren't saying how long airport security measures employed for imminent terrorist threats will stay in place here and in the United States.

"We want to make sure that there are no remaining threats out there, and we also want to take steps to prevent any would-be copycats who may be inspired to similar conduct," said U.S. Homeland Security Secretary Michael Chertoff.

Terror threat levels were raised to the top of their scales and hundreds of flights were canceled worldwide Thursday after British police said they thwarted a terrorist plot, possibly just days away, that would have killed thousands of travelers aboard as many as 10 flights.

"Quite simply, this was intended to be mass murder on an unimaginable scale," said Paul Stephenson, deputy chief of London's Metropolitan Police.

Chilling accounts leaked by investigators described a plan on the scale of 9/11 that would use common electronic devices to detonate liquid explosives concealed in sports drink bottles to bring down as many as 10 planes in a nearly simultaneous strike.

The bombs were to be assembled on the aircraft apparently with peroxide-based solution and everyday carry-on items such as a disposable camera or a music player, two American law enforcement officials told The Associated Press. The officials spoke on condition of anonymity because Britain asked that no information be released.

A federal law enforcement official in Washington said that at least one martyrdom tape was found during ongoing raids across England on Thursday. Such a tape, as well as the scheme to strike a range of targets at roughly the same time, is an earmark of al-Qaida.

British authorities arrested 24 people based partly on intelligence from Pakistan, where authorities detained up to three British nationals several days earlier. The suspects were believed to be mainly British Muslims, at least some of them of Pakistani ancestry.

The Bank of England froze the assets of 19 people early today and released their names, saying they had been arrested on Thursday. All had Muslim names, many of which are common in Pakistan. The youngest person was 17, the oldest 35. Intelligence officials said they believed some plotters were probably still at large.

A senior U.S. counterterrorism official said the suspects were looking to sneak at least some chemicals on the planes in sports drink bottles. Teams of at least two or three men were assigned to each flight, the schedules for which they had researched on the Internet, the official said.