LONDON — British police said Thursday they thwarted a terrorist plot, possibly just days away, to blow up U.S.-bound jetliners over the Atlantic and kill thousands. Chilling accounts leaked by investigators described a plan on the scale of Sept. 11 that would use liquid explosives concealed as everyday carry-on items and common electronic devices to bring down 10 planes in a nearly simultaneous strike.

The bombs were to be assembled on the aircraft apparently using a liquid explosive — most likely peroxide — and detonated by such devices 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 others several days earlier. More arrests were expected, the official said. The suspects were believed to be mainly British Muslims, at least some of Pakistani ancestry.

The official said some suspects were cruising the Internet looking for flights to the United States for several different days in recent weeks. He said some had gone to Pakistan recently.

American investigators praised Britain for preventing a catastrophe. "If this plot had actually occurred, the world would have stood still," Mark Mershon, assistant director of the FBI, told the AP in New York.

Terror threat levels were raised to some of their highest levels and hundreds of flights were canceled worldwide. Passengers stood in line for hours and airport trash bins bulged with everything from mouthwash and shaving cream to maple syrup and fine wine. Governors in at least three U.S. states — California, New York and Massachusetts — ordered National Guard troops to help provide security.

"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.

Experts said the nature of the plot could herald a new age of terrorism where attackers have access to explosives that are easy to carry and conceal. Emergency security measures quickly implemented on Thursday provided a stark vision of the possible future of air travel.