(CNSNews.com) - United Kingdom authorities are calling it the biggest security threat since 9/11: Authorities have smashed what they're calling a significant plot to blow up aircraft in flight.
Speaking shortly before 5 a.m. Washington time, a Scotland Yard official in London called it an "attempt to commit mass murder on an unimaginable scale."
Passengers boarding flights at British airports Thursday were being refused permission to take hand luggage -- and liquids -- onboard, and those flying to the United States faced additional searches.
A number of suspects were arrested overnight in London, Birmingham and the Thames Valley area surrounding Heathrow Airport -- 21 were in custody, Scotland Yard said, with more still at large, indicating that the threat is not yet over.
The arrests followed a joint Scotland Yard-MI5 security service operation that Scotland Yard said had lasted "several months." The U.S. Central Intelligence Agency is said to be involved as well.
"A major terrorist plot to allegedly blow up aircraft in mid-flight has been disrupted in a joint pre-planned intelligence-led operation," Scotland Yard said in a statement.
"It is believed that the aim was to detonate explosive devices smuggled on board the aircraft in hand luggage. It is also believed that the attacks would have been particularly targeted at flights from the U.K. to the U.S.A."
Authorities said four airports were involved: Heathrow in London as well as Manchester and Birmingham, also in England; and the airport in Glasgow, Scotland.
Home Secretary John Reid said the alleged plot aimed to "bring down a number of aircraft through mid-flight explosions, causing a considerable loss of life."
Reid said the country's threat state had been raised early Thursday to "critical," the highest of its five levels. Likewise, the U.S. government has raised its security level on all flights, with those from Britain put on the highest (red) level.
Reid described the beefed up security measure as precautionary, saying the government was doing everything possible to disrupt any further terrorist activity. He also predicted that there would be "major disruption" and significant delays at Britain's airports -- and that indeed turned out to be true.
Heathrow Airport -- which claims to be the busiest international airport in the world, with 67.7 million passengers last year -- was later closed to incoming traffic.
MI5 on its website says the "critical" threat level "means that an attack is expected imminently and indicates an extremely high level of threat to the U.K."
In an unprecedented measure, passengers were being asked to place the items they would ordinarily have taken into the cabin with them into their checked baggage.
Those items they had to keep on their person -- passports, tickets, pocket-sized wallets but not handbags, spectacles, baby food etc. -- had to be carried onboard in a single, preferably transparent plastic bag, according to the Home Office.
All passengers were to be hand-searched, and those boarding flights to the U.S. would have to undergo a secondary search just prior to walking onto the aircraft.
Judging from the regulations announced by the Department for Transport, security officials are particularly interested in liquids.
Prescription medicines required during the flight may be included in the plastic bag, "except in liquid form unless verified as authentic."
Contact lens holders may be taken onboard, but not bottles of liquid lens solutions.
Passengers carrying bottles of baby milk have to taste them in front of security staff.
And when U.S.-bound passengers undergo their secondary search at the boarding gate, "any liquids discovered must be removed from the passenger."
Already a spin-off effect was making itself evident Thursday morning, when numerous airlines temporarily canceled services to Britain.
Scotland Yard urged the public to remain "calm, patient, and vigilant."
In a major speech on Wednesday -- just hours before the news of the plot was made public -- Reid warned Britons: "We are probably in the most sustained period of severe threat since the end of World War II."
Hours later, a Scotland Yard official said the people involved in the plot are criminals and murderers. Although Deputy Commissioner Paul Stephenson would not identify the background of those arrested, he hinted that they were Muslims: "This is about people who might masquerade within a community, behind certain faiths."
Stephenson also said police had been consulting with "community leaders" - an apparent reference to representatives of the Islamic community.
"This is about desperate, desperate people who want to do things that no right-minded citizen of this country or any other country would want to tolerate."
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