A total of 21 people were arrested in pre-dawn raids both in the London area and Birmingham Thursday over the alleged plot, which a senior police officer said was "an attempt to commit mass murder on an unimaginable scale."

Home Secretary Reid said the plot was "very significant" and designed to "bring down a number of aircraft through mid-flight explosions, causing a considerable loss of life."

"Whilst the police are confident that the main players have been accounted for, neither they nor the government are in any way complacent," Reid told a press conference, explaining the decision to raise Britain's security threat level to "critical," the highest level.

Reports in the British media said the plot involved liquid chemical explosives which were to be smuggled aboard in hand baggage, though police have not yet confirmed these reports.

Major disruption at airports

Security was ordered to be tightened at British airports, where departing passengers were not allowed hand baggage except for articles placed in transparent bags. There were also restrictions on fluids. Those travelling with an infant were required, for example, to taste the contents of bottles of baby milk.

Long and chaotic lines formed at British airports, including Heathrow and Stansted, as well as Manchester in northern England.

British Airways cancelled all its short-haul inbound and outbound flights from Heathrow to destinations at home and across Europe until 1400 GMT.

Reacting to news of the foiled plot, German officials also upped security procedures at German airports. Thousands of passengers were stranded due to cancelled or delayed flights. German carrier Lufthansa cancelled all flights into Heathrow on Thursday until 5:00 p.m. local time. Three Lufthansa flights that were en route to Heathrow when the news of the plot broke turned around and flew back to Germany.

German politician calls for more security

In Berlin, the Christian Democrats' deputy parliamentary group leader, Wolfgang Bosbach, spoke of a worrying threat.

"The events in London show that we have to increase our security procedures," Bosbach told Reuters. "We have to determine whether, in a similar situation, we would have discovered the explosive material in baggage checks."

The Social Democrats' interior expert, Dieter Wiefelspütz, however, warned against a frantic debate about security.

"As long as the events in London are still under investigation, we shouldn't start talking about further consequences," he told the Netzeitung.

Months-long operation

The British plot was uncovered after a joint operation by Scotland Yard's anti-terrorist branch and the security services that lasted several months, London's Metropolitan Police said.

The heightened security alert comes 13 months after four British Islamist suicide bombers killed 52 people and injured about 700 on London's transport network.

Last month, al Qaeda called on Muslims to fight those who backed Israel's attacks on Lebanon and warned of more attacks unless US and British forces pulled out of Iraq and Afghanistan.

Independent terrorism expert Paul Beaver said that the nature of the suspected plot suggested a connection to al Qaeda.

"In the last two months al Qaeda promised that it would avenge Iraq and Afghanistan by attacking British and American aviation assets -- I see a direct link with that," he said.