By ALAN COWELL and DEXTER FILKINS LONDON, Aug. 10 — The British authorities said Thursday that they had thwarted an advanced terrorist plot to blow up airplanes flying from Britain to the United States using liquid explosives that would have escaped airport security.
The officials said they had arrested 24 men, all British-born Muslims, who planned to carry the liquids in drink bottles and combine them into explosive cocktails to commit mass murder aboard as many as 10 flights high over the Atlantic.
Intelligence officials said they believed that some plotters were probably still at large, requiring increased airport security.
Airports, which faced chaotic delays and cancellations, instantly changed rules on what passengers could carry on board. In the United States, liquids, gels and creams were banned from carry-on luggage. In Britain, all carry-on items were barred except objects like wallets and eyeglasses without their cases.
Officials said the plot — of which few concrete details were made known — bore the hallmarks of Al Qaeda and involved links to plotters in Pakistan.
Late Thursday, the authorities in Pakistan said an unspecified number of arrests had been made there, too.
An American counterterrorism official, who spoke in return for anonymity because of the sensitivity of the case, said several of the plotters had traveled to Pakistan in the last few weeks and might have met there with at least one person affiliated with Al Qaeda. The official said it was after that person’s arrest by Pakistani authorities that the British, fearing that word of the detainment would send the plotters into hiding, decided to move in.
This is the latest in a series of conspiracies apparently rooted in the disaffection of young, British-born Muslims, many of Pakistani descent, who cast themselves as part of a jihadist struggle against Britain, which they see as an outrider of the United States in Iraq, Afghanistan and now Lebanon.
It also mimicked a failed plot in the Philippines in 1995 financed by Osama bin Laden to blow up airplanes over the Pacific. That ended when the chemicals exploded at an apartment in Manila.
On Thursday, Britain raised its terror threat assessment by one notch to its highest level, “critical,” meaning an attack was imminent.
The American official said the plotters were planning a “dry run” of the operation in the next few days when they planned to test whether they could board flights simultaneously. If this had worked, a full-scale attack would have been carried out within days, the official said.
British police officials, who spoke in return for anonymity because of their customary procedures, said the attacks had not been planned for Thursday.
One American official said the attack was not imminent. “I would caution about how close it was,” he said. “They had materials, but it wasn’t like they were driving out to the airport the next day. They identified a number of flights.”
Peter Clarke, London’s top counterterrorism police officer, said, “The intelligence suggested that the devices were to be constructed in the United Kingdom and taken through British airports.” But he also said that some unspecified event or development late Wednesday convinced British counterterrorism operatives that they must move quickly to thwart a conspiracy with what he called “global dimensions.”
In recent days, the Federal Bureau of Investigation sent hundreds of agents around the United States to chase down possible leads from British intelligence sources.
“There is no indication as of now that anyone in the U.S. was tied to this,” a senior Justice Department official said.
About 8:30 Wednesday night, federal officials called security officers at the major airlines and told generally of what was happening as well as the security measures that would begin on Thursday. Among the airlines believed to be targets were United, American and Continental, according to officials from the Department of Homeland Security, although it was unclear whether the plotters had bought tickets.
Mr. Chertoff said the attackers planned to carry explosive material and detonation components “disguised as beverages, electronic devices and other common objects” onto the planes.
A bulletin issued Thursday by the F.B.I. about the plot gave details of some of the properties of liquid-peroxide-based explosives. It noted that they are sensitive to “heat, shock and friction” and can be detonated with heat or an electric charge.
In some ways, news of a plot that could have killed thousands of people reinforced the sense among Americans after Sept. 11, 2001, and the British after July 7, 2005, that their world had changed irrevocably in a way few would have wished. “This is the new way of life,” said Arleen Malec, 60, a homemaker from Chicago who arrived in London from the United States.
But news of the plot also played into the fractured politics on both sides of the Atlantic, bolstering the arguments of those in London and Washington who argue, like Prime Minister Tony Blair, that the West is locked in an “elemental battle” with radical Islam. In the United States, President Bush said the plot showed that the United States was “at war with Islamic fascists who will use any means to destroy those of us who love freedom.”
At the same time, both American and British officials were left to contemplate what seems a remarkable robustness among British jihadists who, since shortly after the Sept. 11 attacks, have been embroiled in five or six known conspiracies. Some have been unsuccessful, some unproven and one — on July 7, 2005 — terrifyingly effective when four bombers killed themselves and 52 commuters on the London transportation system.
But all of them have defied official British efforts to forestall new attempts, either through ever-more stringent security arrangements that have angered civil rights groups or through efforts to embrace what are seen as moderate Muslim leaders. The latest conspiracy came despite the jailing or forced exile of prominent radical clerics like Sheik Abu Hamza al-Masri and Sheik Omar Bakri Mohamed.
For the first time in the United States, the threat assessment level on transatlantic flights was raised to its highest — “red” — and stringent new security measures were enforced, as was the case in Britain.
The suspects were arrested in nighttime police raids on modest-seeming homes as far apart as East London’s Walthamstow District; High Wycombe, west of the capital; and Birmingham, in the Midlands. Some 24 hours later, none had been identified by name.
[Early Friday, the Bank of England announced that it had moved to freeze the funds of 19 of the suspects, and it released their names, ages and hometowns. The youngest two were 17 and 19, the oldest was 35, and the rest were in their 20’s.
[Scotland Yard had no immediate comment on the bank’s statement, The Associated Press reported.]
The conspiracy, which British officials said had been under surveillance for months, again raised the question of how closely British-born terrorists were linked to Al Qaeda.
Robert S. Mueller III, the F.B.I. director, said the plot had “all the earmarks of an Al Qaeda plot” but added that there was no direct evidence of this.
Also in the United States, Homeland Security Secretary Michael Chertoff said new restrictions imposed on travelers reflected a belief that the plotters planned to use liquids, “each one of which would be benign, but mixed together could be used to create a bomb.”
He added, “It was not a handful of people sitting around and dreaming.”
Referring to the 24 people arrested under counterterrorism laws, John Reid, the British Home Secretary, told reporters that the police were “confident that the main players have been accounted for.”
He acknowledged similarities to the plot in the Philippines 11 years ago. The conspirators in that plot were Khalid Shaikh Mohammed, who was starting his climb to become a top Qaeda operative, and Ramzi Ahmed Yousef, who was the mastermind of the 1993 bombing of the World Trade Center. After the Sept. 11, attacks, another plot involving airplanes related to Richard C. Reid, the so-called shoe bomber, a British-born Muslim who tried to blow up an America-bound flight from Paris with explosives hidden in a sneaker.
The scale of the British raids and arrests was particularly remarkable since, only weeks ago, the police drew criticism from British Muslims for arresting two brothers in East London whom they later had to release for lack of evidence. At the time, the police indicated that they were looking for a chemical bomb.
At Britain’s airports, chaos spread rapidly as airlines closed down flights. For much of the day, British Airways canceled short flights to Europe, and many European airlines canceled flights to London.
Travelers on the Heathrow Express train from London’s Paddington Station heard a recorded female voice announcing: “You can take tissues on the plane, but only if they are unboxed. You can also take baby food and milk on board, but the contents of the bottle must be tasted by the accompanying passenger.”
At Terminal 3 in Heathrow Airport, hundreds of passengers jammed into the terminal building, and airline officials handed out clear plastic bags to passengers for their limited carry-on items. Many passengers said airline officials refused to tell them what was going on.
In Paris, after an emergency cabinet meeting, Interior Minister Nicolas Sarkozy said France had increased its security alert to “red” — one degree below maximum level. He said France had stepped up airline security measures and was introducing new steps for flights bound for Britain and Israel, including thorough searches of all hand baggage.
French airports were nonetheless crowded with thousands of stranded passengers. Flights to Britain were canceled during most of the day, but air traffic bound for the United States was close to normal, spokesmen for different airlines said.
Many passengers deprived of their connections from London flocked to the Eurostar train under the Channel, to reach the European continent and to continue their travel via Paris. At train stations and airports, uniformed members of the police and military increased their patrols.
In Spain, only about 10 percent of 800 flights to London took off, leaving thousands of vacationers stranded, according to Aena, the main operator of Spanish airports. The Prime Minister, José Luis Rodríguez Zapatero, told reporters that the Spanish authorities had “ordered the tightening and strengthening of all the controls that affect the security of the airports in our country.”
Prime Minister Blair had left for a Caribbean vacation last weekend after delaying his plans because of the Lebanon crisis. His office said that he had briefed President Bush on the situation. British politicians in general refused to be drawn into what has become a familiar public debate about whether Britain is a target for terrorists because of its alliance with the United States.
In High Wycombe, west of London, residents said the police seemed to have raided two places and a wooded area. “It’s shocking when it’s so close to us, on our doorstep, not on the other side of the country,” said Sue Needham, a 36-year-old homemaker near the wooded area.
In Walthamstow, East London, John Weir, 50, said he lived opposite one of the houses raided in London. “It was sold overnight,” he said. “One day it was up for sale, and the next it was gone.” He said two men moved in the next weekend, but the house often seemed empty.