Describing the plot as an attempt to cause "mass murder on an unimaginable scale," they said arrests had been made in the London area and in Britain's second city Birmingham.

British police hinted Thursday that British Islamists were behind a major terrorist plot to blow up airlines flying to the United States.

Deputy Commissioner Paul Stephenson, from London's Metropolitan Police, stressed that the plot concerned "people who might masquerade within a community behind certain faiths".

The term "community" is often used in Britain to refer to people from the country's minority faiths and ethnic groups, particularly the Muslim population.

The majority of those arrested were understood to be of Pakistani origin but most, if not all, were British, Britain's domestic Press Association news agency said, citing unnamed senior police sources.

Police said they had foiled the plot, which involved smuggling explosive devices on board planes in hand luggage in an attempt to cause mass murder "on an unimaginable scale".

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" device.

"The police acting with the security service MI5 are investigating an alleged plot to bring down a number of aircraft through mid-flight explosions, causing a considerable loss of life," Interior Minister John Reid said.

The plot to blow up airliners targeted at least three major US carriers on flights from Britain, with destinations that included at least five major US cities, a US intelligence official said Thursday.

The official, who spoke on condition of anonymity, said United, American Airlines and Continental Airlines were among the carriers targeted although the targets might also have included non-US carriers.

The destination of the targeted flights included cities that were "primary tourist attractions," including New York, Washington DC, and Los Angeles, the official said. "You'd have to add Boston, you'd have to add Chicago. Kind of like the big hubs if you will," he added.

The US Department of Homeland Security raised the threat level for all commercial aircraft to high and US authorities banned liquids, including drinks, from all US commercial flights.

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."

The British Airports Authority said it had asked all European carriers to suspend flights to London's main Heathrow airport where new security measures caused severe delays.

The news came amid high international tension over the war in Lebanon and the week British Prime Minister Tony Blair headed on holiday to the Caribbean.

London police said they believed the intention was to target flights from Britain to the United States.

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.

Blair's office said the prime minister had briefed President Bush on the operation during the night and had been in constant contact with Britain over the situation.

Reid said it was a "very significant plot."

"We are doing everything possible to disrupt any further terrorist activity," he said in a statement. "This will mean major disruption at all UK airports."

Air passengers found they could take little on board.

"We cannot take on anything except wallets, passports and medicine," Dana Cojocaru-Ivoska, 28, trying to get on a flight to return to her home in St. Louis, Missouri.

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

In a speech on Wednesday, Reid said Britain was in the most sustained period of severe threat since the end of World War Two and warned there was no room for complacency.

A police source said more than 15 people had been arrested.

"We don't think that it was planned to happen today," the source told Reuters. "We had intelligence and we had to move against what was a planned attack.

"The plan was to take a ready-made explosive device rather than something which would be made up on board," the source said.

Independent terrorism expert Paul Beaver said hand luggage was a weak spot in airport security.

"A laptop computer can carry enough explosives to blow up an aircraft," he said. "Hold baggage and cargo can be sniffed for explosives. You can't do that for hand luggage at the moment. The technology is there, but it's time consuming and expensive."

Beaver said the nature of the alleged 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.

Britain has come under fire from Islamist militants for its military campaigns in Iraq and Afghanistan.

Blair has also come under strong criticism at home and abroad for following the US lead and refusing to call for an immediate ceasefire in the conflict between Israel and Lebanese Hezbollah guerrillas.

British Airways said no hand baggage would be allowed on any of its planes leaving British airports. It said no electrical or battery powered items would be allowed in the cabin, including laptops and mobile phones.

British Airports Authority said all passengers on flights to the United States would be subject to a secondary search at the boarding gate and all liquids would also be removed.

The Home Office Web Site showed the threat rating had been raised to "critical" from "severe" earlier on Thursday. Britain has a five level rating for security threats, ranging from low to critical, which is the highest level.