CHICAGO — Long lines of irritated travelers snaked through U.S. airports Thursday as people waited hours to reach security checkpoints, then had to dump their water bottles, suntan lotion and even toothpaste following the discovery of a terror plot in Britain.

Guards with rifles stood watch in several U.S. airports, and the governors of California, New York and Massachusetts sent National Guard troops to bolster security.

The hours-old ban on all liquids and gels from carry-on luggage left travelers Thursday with little option but to throw away bags of makeup, perfume and bottles of liquor and wine. Baby formula and medicines were exempt but had to be inspected.

"They're ridiculous, but that's part of the price you pay for traveling during a time like this," Julius Ibraheem, 26, a college counselor from Chicago, said as he stared at the long lines leading toward the checkpoints at O'Hare Airport.

At Baltimore/Washington Airport, security workers opened every carry-on bag that passed through one terminal, and all the morning flights there were delayed.

"It's better alive than dead," said Bob Chambers, whose flight from Baltimore to Detroit for business meeting was delayed more than an hour. "It's inconvenient, but we'll make it."

The plot targeted flights from Britain to the United States, 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.

U.S. authorities raised the terrorism threat level to "red" for flights from Britain, the first time the highest threat had been invoked since the system was created. All other flights were under an "orange" alert — one step below red. Homeland Security Secretary Michael Chertoff said the plot appeared engineered by al-Qaida, the group blamed for sending hijacked planes into the World Trade Center on Sept. 11, 2001.

Despite the frenzy of attention in Britain and the U.S., passengers arriving on a flight from London's Heathrow Airport to Washington's Dulles around 1 p.m. Thursday were unfazed, though about two hours late.

"It was really calm, everyone was calm," said Jim McConnell of Charlottesville, Va., who had been on the United Airlines flight. "I think people have grown to accept the state of the world."

Tempers were more heated in the long checkpoint lines that ran in switchbacks across San Francisco's United Airlines terminal. Red plastic bins quickly filled up with confiscated water bottles, and confused travelers pushed and elbowed one another.