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 Qaeda, 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.

Kathy McMahon, 49, of Mill Valley, Calif., was frantically helping her daughter stuff sunscreen, makeup, contact lens solution and other liquids into every corner of her half-dozen suitcases to be checked as she headed off to college.

"I think it's ridiculous," McMahon said. "But we'll do it anyway. What are you going to do?"

At Kennedy Airport in New York, Sonia Gomes De Mesquita, 40, waited nervously to board a British Airways flight home to London. Her family had urged her not to fly.

"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.

Andra Racibarskas, of Chatham, was trying to get to Michigan to pick up her daughter from camp.

"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."

As the morning went on and more passengers became aware of the ban, the long wait in security lines at Atlanta's airport dropped from what at one point became a two-hour delay, airport spokeswoman Felicia Browder said.

Some passengers gave banned items away. Airport officials in Manchester, N.H., officials offered padded envelopes and paid the postage to mail items home.

At the 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.

In Atlanta, Brenda Lee was annoyed with the airport lines and having to remove items from her luggage. The 52-year-old real estate appraiser from Snellville, Ga., had to throw away her shampoo, but she said she was keeping her contact lens solution.

"I'm not sure it does what they want it to do," she said. "It's all for security, but some things go beyond security."

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."

Chicago aviation commissioner Nuria Fernandez said the tighter restriction will remain in place for at least 12 to 72 hours and possibly longer.

At Boston's Logan Airport, Gov. Mitt Romney said additional screening stations were being set up at the airline gates and security was being tightened on the roads outside the airport. The exact number of National Guard troops was still being determined, but "it will certainly be in the hundreds," he said.

California Gov. Arnold Schwarzenegger said he was sending bomb-sniffing dogs with the California National Guard and Highway Patrol to help with airport security.

In New York, Gov. George Pataki said he was increasing the National Guard presence, though New York City Mayor Michael Bloomberg said he didn't think it would be necessary. "The nature of this plot doesn't lend itself to the high-profile police presence" that followed the London subway bombings, Bloomberg said.

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 1 1/2 to 3 1/2 hours late. The cancellations were due to scheduling delays and not because of direct threats to the flights, Hotard said.

Industry analysts said the immediate impact for airlines would be the cost of canceled flights, but in the long run, all airlines would face higher security costs.

In line at Los Angeles Airport, Minneapolis-bound passenger Sunita Reddy, 25, applied a few final dabs of lotion to her hands and face, then dropped the tubes into a large blue trash bin, resigned to the changing state of air travel security.

"I think it's OK," she said. "It's for the public good."