British authorities said Thursday that they had disrupted a terror plot by British citizens of Pakistani origin with ties to al-Qaeda to blow up several airplanes while in flight from the United Kingdom to the U.S., arresting the “main players” in the scheme and raising security at airports.

The plot involved having terrorists carry liquid explosives disguised as beverages and other necessary materials onto planes and assemble them in flight. Home Secretary John Reid said that the plan was to blow up several planes, as many as ten, “through mid-flight explosions, causing a considerable loss of life.”

The plan was “quite close” to being executed, Homeland Security secretary Michael Chertoff said.

Police arrested 21 people in London, High Wycombe and Birmingham in connection with the plot, though are not entirely sure they have caught everyone who may be involved.

A senior U.S. counterterrorism official said authorities believe dozens of people — possibly as many as 50 — were involved in the plot. The official spoke on condition of anonymity because of the sensitivity of the situation. — Associated Press

“This wasn’t supposed to happen today,” said a U.S. official who asked not to be identified. “It was supposed to happen several days from now. We hear the British lost track of one or two guys. They had to move.” — Washington Post

MI5 have raised the country’s terrorism alert level from severe to critical, its highest level, while the U.S. Department of Homeland Security has raised aviation alert levels to High (orange) for all flights, except those from the U.K. to the U.S., where the alert level is now Severe (red).

People traveling in the U.S. will have to deal with new security restrictions, including a ban on liquids and gels in carry-on baggage, except for baby formula for children. Security officials in the U.K. have been asking passengers to taste the baby formula before allowing it to pass.

Officials say they hope the new restrictions will remain in effect only for a limited time.

Chertoff said that additional air marshals would be deployed to the U.K. in light of the threat. Homeland Security will also be introducing “the use of advanced targeting tools, special response teams including baggage and aircraft search teams, baggage X-ray equipment, specially trained K-9 units and explosive detection technology,” he said.

Advanced targeting tools means data-mining. And we all know how well that’s worked in the past.

Massachusetts Gov. Mitt Romney wasn’t taking any chances. He dispatched the National Guard to Boston’s Logan Airport.

Hours-long delays have been reported at all major U.S. airports as people are being forced to throw away soda, suntan lotion, makeup, perfume and toothpaste.

“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.”

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, were even worse. The crush of people brought to mind a chaotic rock concert. — Associated Press

This is not the first time terrorists have attempted to use liquid explosives to blow up airliners. The 1995 Bojinka plot which was disrupted by Philippine and U.S. authorities planned to blow up airliners in flight using explosives smuggled on board in bottles of contact lens cleaner.