LONDON — British authorities said yesterday that they have disrupted a sophisticated and well-advanced terrorist plot to blow up American-bound airliners using liquid explosives, arresting 24 people who planned to commit mass murder over the Atlantic Ocean, police said.

Police said they were confident that they had the “main players” in custody, but authorities in London and Washington ordered a full-scale security clampdown at American and British airports, out of concern that other plotters may still be at large. A number of trans-Atlantic flights from Britain were canceled, others were delayed, and new security procedures caused major tieups at American airports as authorities enforced a ban on bringing liquids aboard planes in carry-on luggage.

In America, the Department of Homeland Security raised the terrorist threat level to “Code Red,” the highest alert stage, for American-bound flights originating in Britain, and to “Code Orange,” the second-highest level, for all other commercial flights operating in or destined for America.

According to an American intelligence official, the plotters were “days away” from going through with their plan. Searches Wednesday night turned up airline schedule information in their possession, the official said, speaking on condition of anonymity. All of those arrested were British citizens, and most appeared to be of Pakistani descent, the official said.

Another American intelligence official said some of those arrested had traveled to South Asia recently and that the Pakistani intelligence service had been helpful in the investigation. The alleged plotters “were very, very far along,” and their plan had progressed to the point “where it could have been carried out in the very near future,” the official said. “I’m talking about days or a few weeks. This month.”

British authorities initially reported that 21 suspects had been arrested. Late yesterday, Scotland Yard increased the figure to 24.

Pakistani authorities also made “some arrests” of British nationals in connection with the arrests in Britain, a spokeswoman for the Pakistani Foreign Office, Tasnim Aslam, said. She declined to elaborate on the number of people arrested or their identities.

A senior European intelligence official said at least a few people suspected of being involved in the plot were missing and that it was unclear whether they were still in Britain. Investigators also are still trying to determine who was running the operation and whether that person was based in Pakistan, the official said. The plotters had not yet booked specific flights but were searching airline routes and apparently did not plan to purchase tickets until the last minute, the official said.

The FBI chased numerous leads over the last several weeks related to the case, including running down “communications” involving some suspects and people in America, a senior American law enforcement official said. But the official insisted that no evidence has been found of plotters or accomplices in America.

British authorities said the threat involved terrorists who aimed to smuggle liquid explosive material aboard airplanes in hand baggage, as well as timers and detonators that could be assembled in flight. Britain’s home secretary, John Reid, said the operation was aimed at bringing down “a number of aircraft,” reportedly as many as 10, “through mid-flight explosions, causing a considerable loss of life.”

President Bush called the alleged plot “a stark reminder that 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.” In brief remarks on an airport tarmac in Green Bay, Wis., en route to deliver a speech about the economy, Mr. Bush said America is “safer than it was” before the September 11, 2001, terrorist attacks, “but, obviously, we’re still not completely safe because there are people that still plot and people who want to harm us for what we believe in.”

He said, “Travelers are going to be inconvenienced as a result of the steps we’ve taken,” adding, “I urge their patience and ask them to be vigilant.”

The Homeland Security secretary, Michael Chertoff, said at a news conference yesterday in Washington that the plot was “well-planned and well-advanced.” He said the plotters “had accumulated and assembled the capabilities that they needed and were in the final stages of planning before execution.”

Mr. Chertoff said the plotters were “very determined and very skilled and very capable” and that their plan was “certainly about as sophisticated as any we’ve seen in recent years, as far as terrorism is concerned.” He said, “This was not a circumstance where you had a handful of people sitting around coming up with dreamy ideas about terrorist plots.”

Mr. Chertoff said that, although the plan was indicative of an Al Qaeda operation, American and British officials “cannot yet form a definitive conclusion” about Al Qaeda involvement.

London’s deputy police commissioner, Paul Stephenson, said the arrests in London, southeastern England, and Birmingham, England’s second-largest city, came after a months-long investigation into what he said was a plan for “mass murder on an unimaginable scale.”

The chief of the London police department’s anti-terrorism branch, Peter Clarke, said the investigation reached a “critical point” Wednesday night, requiring immediate disruption of the plot, the arrests, and the imposition of heightened security measures.

The plotters intended to target flights operated by American Airlines, Continental Airlines, and United Airlines, an American official said.

Passengers at all airports in America were told to expect intensified searches, considerable delays, and new restrictions on carry-on items. The Transportation Security Administration announced that passengers on all American flights, domestic and international, would be banned from transporting any type of liquid or gel in their carryon luggage.The ban applies to all types of beverages, shampoo, toothpaste, hair gels, and other items of a similar consistency, the TSA announced.

Mr. Chertoff said there were exceptions for baby formula and medicines but that these would be closely inspected at airport security checkpoints.

The hastily imposed new rules forced passengers at American airports to dump their water bottles, suntan lotion, perfume, alcohol, and other liquid items before being allowed to board their flights. American airport officials asked passengers to put such items in checked baggage instead.

Mr. Chertoff, who was briefed by British officials, said the measures were taken “as a precaution against any members of the plot who may still be at large” and to “prevent any would-be copycats who may be inspired to similar conduct.”

Mr. Chertoff said the attackers planned to carry explosive material and detonation components on planes “disguised as beverages, electronic devices, and other common objects.” He indicated that the operation might have been difficult to detect if it had reached the execution phase because the plot apparently involved having different operatives separately bring components of a bomb aboard each plane. He said each of the components “would be benign but, when mixed together, would create a bomb.”

The plan, Mr. Chertoff said, was “reminiscent” of a plot hatched in the 1990s by Al Qaeda’s top operative, who masterminded the September 11 attacks, Khalid Sheik Mohammed, to detonate liquid explosives on 11 American airliners over the Pacific Ocean.

A senior American law enforcement official said that, as in the earlier scheme, code-named Bojinka, the alleged plotters in Britain apparently planned to detonate their bombs in midair. “The similarities to Bojinka are striking and are very much on everyone’s minds,” the official said.

That Bojinka plan was foiled when the plotters accidentally started a fire in a Manila apartment in January 1995 while mixing chemicals to make explosives. They had planned to plant bombs on the airliners, disembark at stopovers, and use timers to set off the explosives while the planes were over the Pacific.

The ringleader was Ramzi Yousef, who was later jailed in America for that plot and a 1993 attempt to blow up the World Trade Center in New York. Yousef tested that type of bomb in 1994, when he planted a smaller version on a Philippine Airlines flight bound for Japan, killing a Japanese businessman and injuring 10 other people. Investigators later discovered that Yousef had smuggled nitroglycerin aboard the plane in a contact lens solution bottle.

In 1987, a liquid explosive is believed to have brought down a South Korean passenger plane, killing all 115 on board. According to a suspect’s confession, North Korean agents planted the bomb, which was concealed in an alcoholic drink bottle and detonated remotely.