**Scroll down for many updates and links to others blogging the story**
British authorities thwarted a terrorist plot to blow up several aircraft mid-flight between the United States and Britain using explosives smuggled in hand luggage, officials said Thursday.
"A major terrorist plot to allegedly blow up aircraft in mid-flight has been disrupted in a joint, pre-planned, intelligence-led operation by the metropolitan police anti-terrorist branch and security services," a police spokesman said.
Britain's Home Secretary John Reid said the alleged plot was "significant" and that terrorists aimed to "bring down a number of aircraft through mid-flight explosions, causing a considerable loss of life."
Police arrested a number of people overnight in London after a major covert counterterrorism operation that had lasted several months, but did not immediately say how many.
The national threat level was raised to critical -- a warning level that indicates the likelihood of an imminent terrorist attack. The threat rating was posted on the Web site of Britain's MI5 -- the British domestic spy agency.
The U.S. government raised its threat level to the highest level for commercial flights from Britain to the United States early Thursday.
"We believe that these arrests (in London) have significantly disrupted the threat, but we cannot be sure that the threat has been entirely eliminated or the plot completely thwarted," said U.S. Homeland Security Secretary Michael Chertoff.
Prime Minister Tony Blair, vacationing in the Caribbean, had briefed President Bush on the situation overnight, Blair's office said.
The Department of Transport advised all passengers that they would not be permitted to carry any hand baggage on board any aircraft departing from any airport in the country.
On MSNBC an expert being interviewed made the point that the amount of explosives necessary to bring down a plane is quite small and the components, many liquid, could be easily disguised as cosmetics or other items, such as drinks, that are normally allowed on flights. The point was made that the shoe bomber's bomb was small enough to fit into a shoe.
I had several thoughts upon hearing the news. The first, unfortunately, was that my husband will be flying on Friday. I don't know yet how this will affect domestic flights, but I am assuming there will be heightened security of all flights. The second thought, also unfortunately, was to wonder how long it will take before this plot is somehow traced to Karl Rove as part of a pre-election strategy. (Don't laugh. You all know it is going to happen.) My third thought was about what a small amount of chemicals it takes to kill a bunch of people and how incredibly easy it would be to hide such small stashes of chemicals in baggage, or, say, a country of 25 million or so people.
Update 4:10 -- British Airport Authority News Conference -- a very long list of items that will not be allowed in the plane's cabin. Women's purses will not be allowed. No beverages, hair gels, lotions, etc. Only the absolute necessities will be allowed including money, credit cards, identification, prescription drugs, bottles of formula for infants (but they will be subject to taste testing).
Update II: Fox News has good coverage through their sister station, Sky News, with live interviews with passengers at airport in UK. Will be airing a Scotland Yard briefing at 4:30 a.m. ET.
Additional facts trickling in reported on Fox: the plot was 8 or 9 months in planning and included as many as six planes from four airports including London, Glasgow, and Manchester.
Sky News reporter in one interview tried to make connection between government policy and terrorist plots. He points out British Home Secretary speech from previous day about the effect of terror threat on liberties was made when he had to have been aware of the investigation into this plot.
Update 4:30 -- Fox News reports that terrorists were "not yet sitting on an airplane" but they were very close to traveling. They also are getting some reports that the number of arrests may be higher than 20 and could be as high as 50 and that there is a "footprint" to al Queda. At least two suspects believed to be involved with the plot are still at large.
Update 4:50 -- Scotland Yard News Conference
Repeated basic info already reported above. Significant number of people involved. 21 people in custody. Extraordinarily serious plot which would have resulted in high number of deaths if not thwarted. Would not confirm where these terrorists come from at this time. Wanted to assure public that the inconvenience experienced by passengers is completely warranted. Said this is not about communities, but about mass murderers masquerading in communities.
Update 5:10 -- Fox says this was an international investigation and that Tony Blair had discussed it with President Bush on several occasions, both by phone and in person.
Update 5:20: Several reporters are saying this thwarted attack had the potential of being bigger than 9/11. Scotland Yard news conference clip that is being rerun says this promised to be "mass murder on an unimaginable scale." After 9/11 it is hard to imagine what could have been described as "unimaginable". There were six planes said to be involved. A plan to blow up six planes full of people is horrible, but I don't see how it could be said to be worse than 9/11, which included attacks on towers containing thousands of people and an attack on the Pentagon. I wonder what else was involved here.
Brian Kilmeade on Fox quoted someone making a reference about this plot being in retaliation for Afghanistan and Iraq. Yeah, until Bush decided to use the U.S. military terrorists had no interest in mass murder. (I have a feeling those denying 9/11 will soon surpass those denying the Holocaust. We are coming up on the fifth anniversary of 9/11 and so many have forgotten already.)
Update 6:00 -- E.D. Hill just said what I did above wondering how this could be worse than 9/11 unless something else was involved. Now I am hearing that at least six planes were to be involved, and possibly as many as twenty planes. Maybe that is where the "worse than 9/11" talk comes from.
U.S. authorities are saying the attack was "disrupted" rather than "thwarted."
Update 6:15 -- Counterterrorism officials have told Fox News that the airlines targeted were American, United, Continental and British Airways. There are as many as 50 people still being sought in this matter. No news yet whether those Egyptian students who entered the US (some arrested and some still being sought) have anything to do with this story, but Fox & Friends is asking the question. This Examiner editorial takes a look at the disappearance of those Egyptian students.
Update 7:12 -- The home of a suspect is being searched near Heathrow Airport in High Wycombe.
The investigation is being described as global and it has been reported that MI-5 and the CIA worked together on it.
Update 7:27 -- Regarding my comment above about attributing the plot to Rove campaign strategy? It didn't take long. They started before I even asked.
Update 8:40 - Kim -- Drudge has a headline (no link yet) which says that a report indicates that those involved are citizens of Pakistani decent. Simon Henderson on Fox News says that British police or politicians are loathe to call it Islamic terrorism. Simon says he's willing to money on it that today's terrorists are British Muslims.
Update 9:03 - Kim -- I'm sure others have covered this already, but the explosive was a liquid, completely different from anything else used up to this point, which is why only medicine and baby formula are allowed on. Could this have been nitroglycerine. a la Khalid Sheik Muhammad in 1996?
Update 9:07 - Kim -- Mary Katharine Ham is following the nutroots and can you believe they are questioning the timing of the announcement of the plot.
Continue following this story at Kim's post here.
Update 2:00 - Some details about the Pakistan connection from the UK Times:
Pakistani intelligence agencies helped the British authorities foil the terror plot to blow up aircraft travelling between Britain and America, highly placed sources in Pakistan said today.
The agencies have been working closely with British anti-terror police in monitoring the activities of the suspected terrorists for some time, many of whom have links with Pakistan-based Islamic militant groups, The Times has learnt.
Today Pakistani security forces put Hafiz Mohammed Saeed, leader of the outlawed Islamic militant group Lashkar-e-Taiba, (LeT) under house arrest. The largest of the separatist groups fighting the Indian forces in Kashmir, the LeT has also been blamed by Indian authorities for last month's train bombings in Bombay which killed more than 200 people.
Throughout the night and early morning police carried out a series of raids in London, Buckinghamshire and Birmingham, arresting 21 people.
Three of the properties were in High Wycombe, Buckinghamshire and earlier this afternoon police extended their cordon around one of the homes, a three-bedroom semi-detached ex-council, and then evacuated people from nine homes in the street.
A spokeswoman from Scotland Yard said: "We have evacuated a small number of houses in High Wycombe as a precautionary measure."
Locals said that the family that had lived at the targeted home for the past 15-years included three sons and two daughters.
Neighbours said two of the brothers, who are both married, still live at the house with their wives, but a third brother no longer lives there.
Former friends said that the men used to be well known in the area but had not been seen very much in recent years.
Philip Redfern said: "They are very quiet, they keep themselves to themselves which is very strange because this is a very tight community.
Update 5:15 p.m. - Terrorists planned "dry run" within two days.
Fox News just showed an Air Canada plane on the runway at Washington International. They are reporting that passengers were evacuated from the plane due to a suspicious package. I was unable to find a story about it online at Fox, AP, or anywhere else.
Blogging the story:
Counterterrorism Blog has excellent early commentary.
Tim Blair (who shares Jay Tea's suspicions except that Jay tagged the Lutherans for it)
USS Neverdock (who notes the BBC discovered the "T" word)
Homeland Security Blog (has some really good blogroll links to other national security sites)
Lifelike Pundits (what terrorist threat?)
Michelle Malkin (pulling it all together)
Hot Air (excellent coverage beginning around 1:30 a.m. and continuing through the night)
Ace (great coverage -- lots of stuff)
Flip Pidot (some interesting stuff including last night's Mythbusters episode on liquid explosives)
Mary Katharine Ham (tries to explain to Dems that international terrorist threats should be reported, even if doing so does ruin their Nedrenaline)
Carol Platt Liebau (she has family members who used to fly between the U.S. and U.K. frequently)