In major U.S. airports, guards armed with rifles stood at security checkpoints, and passengers were met by signs warning that all liquids were now banned from carry-on luggage.
Security workers opened every carry-on bag that passed through one terminal at Baltimore-Washington Airport, and all the flights there were delayed.
"It's better alive than dead," said Bob Chambers, whose flight from Baltimore to Detroit for a business meeting was delayed more than an hour. "It's inconvenient, but we'll make it."
The governors of California, Massachusetts and New York called in the National Guard to bolster security.
In the wake of the plot, U.S. homeland security officials raised the threat level for flights between Britain and the U.S. to red, the highest level, for the first time ever, and domestic flights to orange, the second highest level. This will mean big changes for U.S. travelers.
"No liquids or gels of any kind will be permitted in carry-on baggage. Such items must be in checked baggage. This includes all beverages, shampoo, sun tan lotion, creams, toothpaste, hair gel, and other items of similar consistency," a Homeland Security statement said.
The plot in Britain targeted flights from Britain to the U.S., particularly to New York, Washington and California on United Airlines, American Airlines and Continental Airlines Inc., a counterterrorism official said Thursday, speaking on condition of anonymity because of the sensitivity of the situation.
Homeland Security Secretary Michael Chertoff said the plot appeared to have been engineered by al Qaeda, the terrorist group that hijacked two planes from Boston on Sept. 11, 2001, and flew them into the World Trade Center towers in New York.
Security officials stressed they have no knowledge of any current imminent threat against any U.S. flight, and they expressed confidence that the plot to bomb inbound flights from London has been thwarted, reports CBS News correspondent Bob Orr.
Passengers arriving for flights Thursday and discovering the suddenly tightened security clogged the checkpoints as bags were searched, leaving long lines.
At John F. Kennedy Airport in New York, Sonia Gomes De Mesquita, 40, of London, waited nervously to board a British Airways flight to London's Heathrow Airport. Her family had urged her not to get on the plane.
"You wake up and what are you going to do?" she said. "The flight is today."
She said she checked all her belongings rather than risk having something confiscated. "I even checked in my book."
At Newark Airport in New Jersey, the security checkpoint line for Terminal B, home to most international flights, stretched the entire length of the terminal — roughly six football fields — and was barely moving.
"Checking in was very easy. It took one minute curbside. It took one minute to get my boarding pass," she said. "This line is at least four hours long."
The security lines at Newark's Terminal C, where Continental bases its flights at the airport, was even worse. The crush of people brought to mind a chaotic rock concert.
"It's complete disaster and chaos," said Bill Federman, of Oklahoma City, who missed his Continental flight home because of the lines. "This has completely overwhelmed the airport's planning. I haven't seen anything this bad since 9/11."
The new ban on all liquids and gels from carry-on luggage left people with little choice but throw away juice boxes, makeup and, for one passenger, even a bottle of tequila. Baby formula and medicines were exempt but had to be inspected.
Some passengers gave banned items away. Airport officials in Manchester, N.H., officials offered padded envelopes and paid the postage to mail items home.
Laura Yeager left four bottles of Gucci and Cartier perfume for the hotel maid before heading to the Atlanta airport for her flight back to Philadelphia. She still had to give up her lip gloss at the security checkpoint.
She just shrugged and tossed it. "It's better to feel safe. We thought it was going to be a lot worse."
Others were not as easy going. "It's disgusting," one woman told Orr. "I just bought the damn perfume."
At Burlington International Airport in Vermont, travelers weren't happy to be leaving behind souvenir maple syrup jugs. In New Orleans, half-used bottles of hot sauce lay in garbage bins. Bottles of wine sat in the trash in San Francisco, south of California's wine country.
Rather than packing toiletries in carry-ons, airport officials asked passengers to put them in checked baggage, which is screened by equipment that can detect explosives, said Phil Orlandella, spokesman for Boston's Logan International Airport.
Chicago aviation commissioner Nuria Fernandez said the tighter restriction will remain in place for at least 12 to 72 hours and possibly longer.
Extra police and dog units were sent out overnight at Dallas-Fort Worth International Airport, where American Airlines is based, to patrol terminals and parking garages, airport spokesman Ken Capps said.
American canceled three London-bound morning flights from Chicago, Boston and New York to accommodate delays at London's Heathrow Airport, spokesman John Hotard said. To balance the cancellations, the airline also dropped three afternoon or evening flights from London to U.S. cities, Hotard said.
The remaining 13 flights in each direction were expected to run from 1½ to 3½ hours late. The cancellations were due to scheduling delays and not because of direct threats to the flights, Hotard said.
Delta Air Lines spokesman Anthony Black said operations would continue normally and there would be no flight cancellations. But Delta was expecting delays on flights coming from the United Kingdom because of heightened security there, Black said.