A plot to blow up a plane between the UK and the USA was foiled in the UK.
Passengers are only being allowed to carry wallets, passports and other document in clear plastic bags. No bags not even purses.
LONDON (CNN) -- Major delays are expected at all UK airports after the nation's threat level was raised to "critical" Thursday when police disrupted an apparent plot to blow up planes in mid-flight.
The highest possible alert level, raised by the British Home Office, means "an attack is expected imminently," and authorities have said no hand luggage would be allowed onto planes.
London's Metropolitan Police said a months-long intelligence operation by the anti-terrorist branch and security service resulted in several arrests overnight, according to a Scotland Yard news release.
The aim of the alleged plot was to explode devices carried aboard planes in handheld luggage -- especially on flights from Britain to the United States, according to Scotland Yard.
"This is a major operation which inevitably will be lengthy and complex," Scotland Yard said in a statement.
Britain's Home Secretary John Reid called the alleged plot significant and said it was designed to "bring down a number of aircraft through mid-flight explosions, causing a considerable loss of life."
"We are doing everything possible to disrupt any further terrorist activity. This will mean major disruption at all UK airports," Reid said, adding the move to raise the alert level was "precautionary."
Hand luggage will not be allowed into the cabin of a plane and passengers will only be allowed to take a wallet, travel documents and other essential items in a plastic bag.
"All passengers must be hand searched, and their footwear and all the items they are carrying must be X-ray screened," a BAA statement added. (Full story)
Across the Atlantic, the U.S. Department of Homeland Security raised its alert threat to the highest level of "severe," or red, for commercial flights originating in the United Kingdom and bound for America.
Meanwhile, the threat level has been raised to "high," or orange for all commercial flights operating in or coming to the United States, a DHS statement said.
"Due to the nature of the threat revealed by this investigation, we are prohibiting any liquids, including beverages, hair gels, and lotions from being carried on the airplane," the statement said.
Investors were quick to dump airline shares when markets opened on Thursday, with British Airways falling 3.5 percent and Ryanair skidding 3 percent.
The alleged terror plot comes more than five years after Briton Richard Reid attempted to detonate explosives hidden in his sneakers on an American Airlines flight from Paris to Miami in December 2001, only a few months after the September 11, 2001 attacks in New York and Washington.
Passengers thwarted his plan, and the plane landed safely in Boston. Reid pleaded guilty to terrorism charges in October 2002 and is serving a life sentence at the nation's super-maximum security prison in Florence, Colorado.
In Washington, FBI Special Agent Richard Kolko said the agency is aware of the "ongoing situation in London" and will issue a statement later.