LONDON - British police have foiled a plot to blow up aircraft in mid-flight between Britain and the United States, and were holding 21 people after overnight raids, senior officers said.
"We are confident we have disrupted a plan by terrorists to cause untold death and destruction," said London police Deputy Commissioner Paul Stephenson. "Put simply, this was intended to be mass murder on an unimaginable scale."
Both countries stepped up security, causing severe delays at airports following the announcement of the plot, which a police source said was believed to involve a "liquid chemical" device.
Unconfirmed media reports said anywhere from six to 10 US commercial airliners had been targeted in the plot.
The head of London's counter-terrorist police, Peter Clarke, said the plot had "global dimensions". He said the number of flights, destination and timing were still under investigation.
The US Department of Homeland Security raised the threat level for all commercial aircraft to "high" and US authorities banned liquids, including drinks, from US commercial flights.
Britain's security services upped the threat level in the country to "critical" from "severe", the highest of its five ratings, which means "an attack is expected imminently".
The security alert comes 13 months after four British Islamist suicide bombers killed 52 people and injured about 700 on London's transport network.
The nationality of those being held was not immediately known. Arrests were made in the capital London, the southeast of England and Britain's second biggest city Birmingham.
Last month, the global militant group 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.
Al Qaeda hijacked passenger aircraft in September 2001 to destroy the World Trade Centre in New York, and Briton Richard Reid was arrested in December 2001 for trying to blow up a plane headed to the United States.
"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," said Peter Neumann, director of the Centre for Defence Studies at London's King's College university.
"If this had worked out it would have been beyond any doubt the biggest terrorist plot in the UK," he told Reuters.
The British Airports Authority said it had asked all European carriers to suspend flights to London's main Heathrow airport, where tighter security measures caused severe delays.
Shares in European airlines fell on the news, with British Airways shares opening nearly 5 percent lower. The pound also fell against the dollar and the euro. Oil prices fell more than $1 towards $75 a barrel.
Airlines said no hand baggage would be let on planes leaving British airports, banning electrical or battery powered items. Passengers had to hand over all liquids, including contact lens solutions, on flights to the United States.
Neumann said the plot appeared similar to a 1995 plan to blow up 11 planes using nitroglycerine mixed in contact lens solution and a battery powered detonator.
Long queues formed at London's Heathrow and Gatwick airports as passengers were subjected to thorough searches.
"I'm an ex-flight attendant, I'm used to delays, but this is a different kind of delay," said Gita Saintangelo, 54, an American returning to Miami.
"We heard about it on the TV this morning. We left a little early and said a prayer," she said at Heathrow.
Britain has been criticised by Islamist militants for its military campaigns in Iraq and Afghanistan.
Prime Minister Tony Blair has also come under fire at home and abroad for following the US lead and refusing to call for an immediate ceasefire in the conflict between Israel and Lebanese Hizbollah guerrillas.
In a speech on Wednesday, Interior Minister John Reid said Britain was in the most sustained period of severe threat since the end of World War Two.
Blair's office said the prime minister, who is on holiday in the Caribbean, had briefed US President George W. Bush on the operation during the night.
Independent terrorism expert Paul Beaver said 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.
* Have you been affected? If you or anyone you know has been affected by the extra security put in place in Britain today, please Email the Herald news desk