British Home Secretary John Reid said the threat posed by the plot to "bring down a number of aircraft through mid-flight explosions" would have caused "a considerable loss of life." The would-be attackers had targeted as many as 10 planes and were apparently close to carrying out their plan.
"The police believe the alleged plot was a very significant one indeed," Reid said. "At 2 a.m. this morning the Joint Terrorism Analysis Center raised the UK threat state to its highest level critical."
The police have taken at least 21 people into custody in London and elsewhere in Britain after a major anti-terrorist operation lasting several months. The suspects, reportedly from Britain's sizable Asian Muslim community, apparently wanted to use some sort of liquid chemical devices to cause explosions.
"We don't think that it was planned to happen today," a police source told told Reuters. "We had intelligence and we had to move against what was a planned attack. The plan was to take a ready-made explosive device rather than something which would be made up on board."
The incident has created havoc at Britain's major airports, as the authorities are forbidding passengers from taking almost any hand luggage onboard.
Electronic items like laptops, mobile phones and iPods were all coming in for particular scrutiny. The BBC reported that passengers were being giving transparent plastic bags for only the most essential items such as wallets and passports. Several flights going in and out of Britain were cancelled on Thursday and officials in the United States also raised the security threat level for commercial aircraft.
Increased threat level
US Department of Homeland Security Secretary Michael Chertoff said the threat level for commercial flights originating in the United Kingdom would be raised to an unprecedented "severe" level or color-coded red.
"Currently, there is no indication ... of plotting within the United States," Chertoff said. "We believe that these arrests have significantly disrupted the threat, but we cannot be sure that the threat has been entirely eliminated or the plot completely thwarted."
British Prime Minister Tony Blair, currently vacationing in the Caribbean, briefed US President George W. Bush on the situation overnight, according to the Associated Press.
A senior US counterterrorism official said authorities believe dozens of people -- possibly as many as 50 -- were involved in the overseas plot. The plan "had a footprint to al-Qaida back to it," said the official, speaking to the AP on condition of anonymity because of the sensitivity of the situation. British police have not ruled out a link to al-Qaida, but downplayed any direct involvement.
Chaos for air travelers
At London's Heathrow airport, Europe's busiest, check-in lines stretched out the door and armed police searched the terminals. The airport, which normally sees 1,250 departures and arrivals a day, was closed to arriving European flights, and airlines including Alitalia, Lufthansa, Iberia and Aer Lingus canceled flights to Heathrow.
Tony Douglas, Heathrow's managing director, said the airport hoped to resume normal operations Friday, but that passengers would still face delays and a ban on cabin baggage "for the foreseeable future."
"At this point in time it is unclear how long these restrictions will remain in place," he said according to the Associated Press. Some flights were leaving Thursday, but with severe delays.
But many travelers appeared to accept the hassles involved with flying these days. "I'm sure enough people thought 9/11 could have been today. That could have been us," Luci Mason, whose flight to Scotland was canceled, told the AP. "The people here are annoyed, but not up in arms. Some disruption might save my life."