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.

A British Airways aircraft lands at Heathrow Airport, near London August 10, 2006. British police said on Thursday they had thwarted a plot to blow up aircraft in mid-flight between Britain and the United States and arrested more than 20 people. [Reuters]

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.

Mothers tasted baby food in front of airport security guards to prove it contained no liquid explosives. Liquids and gels were banned from flights. Travelers repacked their luggage in airports, stowing all but the most necessary items in the hold.

Although plots to blow up airliners using liquid explosives are not new, such an attempt was foiled more than a decade ago - the U.S. government has been slow to upgrade its security equipment at airport checkpoints to detect explosives on passengers.

U.S. authorities did not say how long the security measures would last. "We are taking the step of preventing liquids from getting into the cabin to give us time to make adjustments," Chertoff said.

The raids in Britain on Thursday followed a monthslong investigation, but U.S. intelligence officials said authorities moved quickly after learning the plotters hoped to stage a practice run within two days, with the actual attack expected just days after that.

The test run was designed to see whether the plotters would be able to smuggle the needed materials aboard the planes, the officials said, speaking on condition of anonymity. Security experts warned it was possible attackers could hide their bomb ingredients in containers for talcum powder or medicine bottles and then assemble the weapon once behind a locked restroom door.

U.S. counterterrorism officials said United, American, and Continental airlines bound for New York, Washington and California were targeted by the terrorists. Los Angeles Mayor Antonio Villaraigosa said the individuals plotted to detonate liquid explosive devices on as many as 10 aircraft bound for the United States.

"This nation is at war with Islamic fascists who will use any means to destroy those of us who love freedom, to hurt our nation," President Bush declared.

The plane bombings could have come just ahead of the fifth anniversary of the Sept. 11 attacks carried out by al-Qaida. The terror group's leader Osama bin Laden and his No. 2, Ayman al-Zawahri, are believed hiding along the Pakistan- Afghanistan border and have repeatedly issued tapes threatening new attacks.

"In terms of scale, it was probably designed to be ... a new Sept. 11," said Jean-Charles Brisard, a French private investigator who works with lawyers of many Sept. 11 victims. "It involved the same tools, the same transportation tools and devices."

The close call also shifted attention once more to Britain's Islamic community just over a year after the London transit attacks. Three Britons of Pakistani descent and a Jamaican convert to Islam carried out those deadly bombings with a peroxide-based explosive that trained operatives can make using ordinary ingredients such as hair bleach.

In Pakistan, an intelligence official said the arrest of an Islamic militant near that border several weeks ago played a role in "unearthing the plot." The official, who spoke on condition of anonymity, said some suspects arrested in Britain were linked to al-Qaida. However, authorities stopped short of accusing al-Qaida directly for the plot.

A senior Pakistani government official, also speaking on condition of anonymity because he was not authorized to comment on the matter, said "two or three local people" suspected in the plot were arrested a few days ago in the Pakistani cities of Lahore and Karachi.

French Interior Minister Nicholas Sarkozy said the group "appears to be of Pakistani origin," but did not give a precise source for the information. Britain's Home Office refused comment.

A British police official, speaking on condition of anonymity because of the sensitivity of the investigation, said the suspects were "homegrown," though it was not immediately clear if all were British citizens. He said authorities were working with Britain's large South Asian community.

Tariq Azim Khan, the Pakistani minister of state for information, said "these people were born and brought up in the United Kingdom. Some of them may have parents who were immigrants from Pakistan."

Raids were carried out at homes in London, the nearby town of High Wycombe and in Birmingham, in central England. Searches continued throughout the day, and police cordoned off streets in several locations. Police also combed a wooded area in High Wycombe.

Hamza Ghafoor, 20, who lives across the street from one of the homes raided in Walthamstow, northeast of London, said police circled the block in vans Wednesday and that they generally swoop into the neighborhood to question "anyone with a beard."

"Ibrahim didn't do nothing wrong," Ghafoor said, referring to a suspect. "He played football. He goes to the mosque. He's a nice guy."

The British government raised its threat assessment to its highest level - critical, which warns that a terrorist attack could be imminent. The U.S. government, following suit, raised its threat assessment to red alert, also its highest level, for commercial flights from Britain to the United States.