New restrictions, prohibiting any liquids in carry-on luggage, are posted at a United Airlines check-in line at JFK Airport, Thursday, Aug. 10, 2006 in New York. U.S. air travelers poured out liquids, opened their bags for inspection and endured long waits Thursday as airports heightened security and some flights were canceled or delayed after the discovery of a terror plot aimed at airlines traveling from Britain to the United States. (AP Photo/Henny Ray Abrams)

LINTHICUM, Md. --  Growing lines of irritated travelers snaked through U.S. airport terminals Thursday as people waited hours to reach security checkpoints, where they were ordered to dump all liquids _ water bottles, suntan lotion, even toothpaste _ following the discovery of a terror plot involving planes leaving Britain.

Guards armed with rifles stood at the security checkpoints in several airports. Massachusetts Gov. Mitt Romney said he would send the National Guard to help patrol Boston's Logan Airport for the first time since the Sept. 11 terror attacks, and New York Gov. George Pataki was considering a similar move.

Security workers at Baltimore/Washington Airport opened every carry-on bag that passed through one terminal, and all the 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 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.

U.S. authorities raised the threat level to "red" for flights from Britain, the first time the highest threat of terrorist attack 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 to have been engineered by al-Qaida, 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.

Lines were longer than usual at San Francisco International Airport, and red plastic bins were quickly filling up with bottles of wine, cups of coffee and water bottles now banned from carry-on luggage.

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."

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.

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.

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.

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

"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."

At one checkpoint in Pittsburgh's airport, small containers of whiskey and scotch were tossed in among the toothpaste and makeup bottles. Other travelers who heard about the ban in time left their forbidden items behind.

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."

At Boston's Logan Airport, 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 guardsmen was still being determined, but "it will certainly be in the hundreds," he said.

New York Gov. George Pataki has offered state police and National Guard units to bolster security at John F. Kennedy Airport and other airports, but no decision to dispatch them had been made, a Pataki spokesman 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.

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.

Homeland Security staff put up hastily printed signs at Dulles Airport outside Washington warning passengers in red capital letters: "No liquid or gels permitted beyond security."

The United Airlines terminal at San Francisco's airport was a scene of growing unhappiness. Both the security and check-in lines had a number of switchbacks and confused travelers pushed and elbowed each other.

Bill Poland, 61, of Ross was headed to Lake George, N.Y., with his wife and son. He held up a tube of lip balm and shouted to a security officer who told him he couldn't bring it on the plane with him. He said he recently had a cancerous growth removed from his lip and the anti-bacterial ointment was necessary treatment.

"In an hour or two my lips are going to start burning and turning purple. And I've got five to six hours on a plane without this," he said. "This is not something I'm looking forward to."

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Associated Press writers Jay Lindsay in Boston, Nathaniel Hernandez in Chicago, Daniel Yee in Atlanta, David Caruso in New York, Wayne Parry in Newark, N.J., Laura Wides-Munoz in Miami, Jordan Robertson in San Francisco and Laura Jakes Jordan and Katherine Shrader in Washington contributed to this report.