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.