The increased security measures at airports instituted after London authorities disclosed that they had arrested only 24 of some 50 terrorists plotting to detonate liquid explosives on airplanes heading to America could be in place indefinitely, officials said.

Governor Pataki said the Department of Homeland Security’s ban on liquids and gels entering the cabins of airplanes would be in effect until officials decide the country is no longer under threat from this form of terrorism. He said the plot, which authorities characterized as sophisticated and well advanced, could lead to changes in the country’s transportation security protocols.

Passengers arrived at airports yesterday to find flights delayed across the board and security lines long and slow-moving. Many were seen frantically attempting to pack their carry-on bags into luggage to be checked.

The only exceptions to the mandate against liquids on flights were baby formula and medicine, but passengers had to be ready to prove the materials were not a risk, including by tasting the materials in front of guards. In Britain, hundreds of flights were canceled and an outright ban on all carry-on luggage was instituted. Officials said they didn’t know for how long the upgraded security protocols would be in place.

The terminals of John F. Kennedy Airport and the AirTrain were plastered with hastily made signs that said no liquids, aerosol cans, mouthwash, toothpaste, or water bottles would be allowed on planes.

“We believe that the arrests in Britain have significantly disrupted this major threat,” the homeland security secretary, Michael Chertoff, said. “But we cannot assume that the threat has been completely thwarted, or that we have fully identified and neutralized every member of this terrorist network.”

Mayor Bloomberg said he and the police commissioner, Raymond Kelly, had been aware of the ongoing investigation in Britain and had incorporated that intelligence into the city’s counterterrorism strategy. When the plot was announced, the Department of Homeland Security immediately upgraded the terror level for all commercial aviation to orange, the second highest on the scale. For flights from Britain, the level was raised to the highest level: red, or severe. The British upgraded their security level to red, or critical, reserved only for when there is an imminent threat against the country.

Early yesterday morning Mr. Pataki informed the state’s airports and other law enforcement agencies of the foiled plot, urging them to be vigilant against a possible threat.The governor also increased undercover patrol of the airports and National Guard presence at transit facilities. He emphasized that neither New York nor any other American city was a target in the attack.

The mayor, governor, and terrorism officials said the timing of the arrests, which are just more than a month before the fifth anniversary of the September 11 attacks, didn’t appear to be a factor.

Sources said the city had been briefed several months ago on a terrorist threat, but the intelligence didn’t yet indicate the full scope of the plot. A communication earlier this week from the police department, however, carried the weight of a broad plot with dozens of people being sought for arrest, the mayor said.

“We were told that commercial jets departing from Manchester and Heathrow in the U.K. destined for New York and other cities in the U.S. were targeted for mid-air bombing with devices disguised as drinks and other liquids,” Mr. Bloomberg said. “We will do whatever it takes to keep people safe, and if it’s an inconvenience it’s just something that we’re going to learn to have to deal with.”

The New York police didn’t respond with the increased deployments they put in place after the train attacks in London, Madrid, and, most recently, Mumbai, because the threat “didn’t lend itself to that kind of response,” the police department’s chief spokesman, Paul Browne, said.

“We tweak what we’re doing every day to a certain extent,” he said. The department has about 1,000 officers devoted to counterterrorism and collecting intelligence. Nine detectives are stationed abroad to keep the city abreast of worldwide terror threats and tactics, including one at Scotland Yard. Detectives also regularly monitor suspicious sales and storage security of dangerous materials, including household chemicals that could be mixed into bombs or chemical weapons.

The plot against the airlines was said to have included more than 50 people, 24 of whom were arrested in towns near London after late-night raids by counterterrorism officers on Wednesday. The men detained were British born and of Pakistani descent. Police in Pakistan reportedly arrested six men for their role in the plot.

The airplanes the plotters were allegedly planning to blow up all flew American flags. The companies that owned the planes are American Airlines, United Airlines, and Continental Airlines, officials said. American canceled three of its 16 daily flights to London, and United revised ticketing policies for the next week, allowing customers to make changes to their itineraries without fees or other requirements.

The city’s tourism marketing agency, NYC & Company, extended the hours of its information center last tonight to 8 p.m. to deal with the influx of travelers who had their flights canceled, officials said.

British travelers were easily identified at JFK Airport after they began to arrive in the afternoon on the first batch of flights to leave after the announcement of the attacks. They hurried through the terminal, looking tired and hassled, carrying clear plastic bags containing all their belongings. Dozens of journalists approached with television cameras and microphones, questioning them about their experience.

A textile designer coming to New York from London for a convention at the Jacob Javits Center, Margo Selby, said she had to throw away her purse at the airport in Heathrow or be forced to miss her flight. In one hand, she carried a plastic bag with her belongings, in the other, a fistful of coins. It was a small sacrifice compared to the immensity of the threat, she said.

“There was significant evidence that they were going to blow us up with bits of chemicals, so I think it’s better to get rid of the liquids just in case,” she said. “I don’t mind.”

A secretary who lives in Connecticut, Brenda Collins, 56, said she didn’t even know about the attacks until she got on the plane. She was allowed to bring only some ulcer pills, a change purse, a passport, and some tissues on the plane. The pilot informed the passengers of the arrests and the terrorist plot, but no one panicked, she said.

“Being that I landed safely and I’m home on American soil, I don’t mind the delay,” she said.

The method of the attack described by the authorities immediately drew parallels to Operation Bojinka, a failed plot to attack American airplanes with liquid bombs in 1995. A senior analyst at the private intelligence firm Stratfor, Peter Zeihan, said the new plot spelled some good news and bad news.

“Al Qaeda is still creative, still trying to find weaknesses in our security,” he said.”But, the shroud of operational secrecy is fraying.”

“Airplanes are the perfect terrorist tool because they are hostages wrapped up and ready to go,” Mr. Zeihan said. “They will continue to be targets and there is very little that anyone can do to change that. You can’t make it completely foolproof. You can only be vigilant and get the public to help.”