The main players behind an alleged plot to blow up aircraft in mid-flight between Britain and the United States are under arrest, British Home Secretary John Reid said today.
Both countries stepped up security, causing severe delays at airports following the revelation of the plot, which a police source said was believed to involve a "liquid chemical."
London police Deputy Commissioner Paul Stephenson
"The police are confident that the main players have been accounted for," Mr Reid said, adding the operation was ongoing and further arrests might be made.
Police are holding 21 people in connection with the plot, which Mr Reid said would have caused "a loss of life to innocent civilians . . . on an unprecedented scale".
The suspects were arrested in overnight raids in London, Birmingham and the southeast.
Pakistan tonight claimed involvement in those arrests but would not confirm whether any arrests had been made in the country.
In a brief speech in Wisconsin this afternoon, US President George W. Bush described the alleged plot as a "stark reminder" that the United States is "at war with Islamic fascists who will use any means to destroy those of us who love freedom, to hurt our nation."
London police Deputy Commissioner Paul Stephenson said: "Put simply, this was intended to be mass murder on an unimaginable scale."
The US Department of Homeland Security raised the threat level for all commercial aircraft to red alert, and US authorities banned liquids, including drinks, from all commercial flights in response to threat to their citizens.
Britain's security services raised the threat level in the country to "critical" from "severe", the highest of its five ratings - which means "an attack is expected imminently".
US Homeland Security Secretary Michael Chertoff said that, while the operation was centred on Britain, it was international in scope and sophisticated, and involved many people.
"This operation is in some respects suggestive of an al-Qaeda plot, but because the investigation is still under way, we cannot yet form a definitive conclusion," he said.
British police sources did not rule out an al-Qaeda link, but played down direct involvement by the global militant group. Police sources said some of those arrested were British Muslims.
Commenting on the nature of the threat, Director of the Centre for Defence Studies at London's King's College university, Peter Neumann, said: "This liquid explosive type of attack is particularly worrying."
"Planes remain vulnerable and in the coming weeks terrorists will be thinking of something else to do that we have no idea about."
Mr Neumann said the suspected plot appeared similar to a 1995 plan to blow up 11 planes using nitroglycerine mixed in contact lens solution and a battery powered detonator hidden in a shoe.
The British Airports Authority has asked all European carriers to suspend flights to London's Heathrow airport where new security measures caused severe delays.
Air passengers were banned from taking hand luggage aboard flights departing from the United Kingdom. Only passports, keys wallets and non-liquid medicines are allowed aboard and carried in a transparent plastic bag.
Baby milk must be tasted by passengers before being brought aboard.
The 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 attacks unless US and British forces pulled out of Iraq and Afghanistan.
Agencies