British authorities say they have foiled a plan to blow up aircraft flying from Britain to the United States. Stringent security restrictions were immediately ordered at British airports, throwing air traffic in and out of Britain into chaos. 

Passengers wait at check-in desks for their flights at Gatwick Airport in England, Aug. 10, 2006

Police and agents of Britain's security service raided homes in greater London and the city of Birmingham overnight, arresting 21 people in what authorities say was a plot to blow up several U.S.-bound aircraft.

Paul Stephenson speaks to media, Aug. 10, 2006

In London, Deputy Police Commissioner Paul Stephenson said the plotters planned to smuggle explosives on board the airplanes, then detonate them while in mid-flight in what he labeled "mass murder on an unimaginable scale."

Deputy Assistant Police Commissioner Peter Clarke, who is responsible for counterterrorism, said the arrests were the culmination of a long investigation in a wide-ranging case.

"We have been looking at meetings, movements, travel, spending, and the aspirations of a large group of people," he said. "This has involved close cooperation not only between agencies and police forces within the United Kingdom, but also internationally. As is so often the case in such investigations, the alleged plot has global dimensions."

Exactly how far along the plot had actually advanced was not revealed. Deputy Commissioner Stephenson would only say that security authorities felt they had to move quickly.

"It did reach a critical point last night when the decision was made to take urgent action to disrupt what we believe was being planned," he added. "The prime motivation for that action was, as always, public safety. We thought it was absolutely essential to take that action last night."

Michael Chertoff, left, and US Attorney General Alberto Gonzales hold press briefing, Aug. 10, 2006

In Washington, U.S. Secretary for Homeland Security Michael Chertoff said it appeared the plotters were ready to act.

"I would say that this plot was well-advanced, and that they had accumulated and assembled the capabilities they needed, and they were in the final stages of planning before execution," he said.

U.S. officials said the plot had the "hallmarks" of an al-Qaida operation. British officials have made no such claims.

As the plot came to light, flights in and out of Britain, a major air hub, were severely disrupted. Some airlines canceled flights altogether for the day, stranding thousands of passengers.

Air security levels were also raised to their highest levels in both Britain and the United States. In Britain, virtually all hand luggage was banned from airline cabins. Transport Secretary Douglas Alexander regretted the inconvenience it caused.

"Earlier this morning we issued instructions to U.K. airports and the airlines that operate from them requiring that new security measures be put in place, he said. "The seriousness of the threat gave us no choice but to require that these stringent measures be implemented with immediate effect."

Passengers like this one at London's bustling Heathrow airport found themselves caught by surprise by the new security measures.

"We went up to the desk and had to wait, and they told us that we had to pack all of our things and put it away into the hold, and that we could not take anything except our documents onto the airplane," said one passenger. "So we got in the queue [line] from the nightmare, and here we are."

Officials said the security measures will stay in place as long as the situation demands.