A police dog handler stands guard at Manchester Airport. British police said on Thursday that 21 people had been arrested in connection with a foiled plot to blow up aircraft flying from Britain to the United States. (Xinhua/AFP Photo)

    LONDON, Aug. 10 (Xinhua) -- Twenty-one people had been arrested in connection with a foiled plot to blow up aircraft flying from Britain to the United States, British police said on Thursday.

    Deputy Commissioner Paul Stephenson, from London's Metropolitan police, told a news conference the terror plot against airlines was "intended to be mass murder on an unimaginable scale."

    The 21 were arrested in London, its suburbs and in Birmingham and that searches continued in a number of locations, Stephenson said.

    "The operation is ongoing. We have been very successful in arresting those we were targeting but this is a lengthy operation, and no doubt there will be further developments," he said.

    "We think this was an extraordinarily serious plot and we are confident that we've prevented an attempt to commit mass murder on an unimaginable scale," he said.

    Stephenson did not give details on how many aircraft may have been targeted, or which airports might have been involved. He also refused to comment on nationalities of the arrested.

    "This is not about communities. This is about criminals, this is about murderers, people who want to commit mass murder. This is not about anything to do with any particular community," Stephenson said.

    "This is about people who might masquerade within a community, behind certain faiths, but this is about people who are desperate, desperate people who want to do things that no right-minded citizens of this country or any other country would want to tolerate," he said.

    Calling on the public to remain "calm, patient and vigilant," Stephenson said the actions taken were "absolutely necessary" and the security forces were dong everything they can to minimize harm.

    Police in London said earlier on Thursday that they had thwarted a major terrorist plot to blow up aircraft in mid-flight.

    Responding to the incident, Britain's Joint Terrorism Analysis Centre raised the country's security threat level to "critical" from "severe."

    Britain has a five-level rating for security threats, ranging from low to critical, which is the highest level.

    Security at the airports across the country has beefed up, and massive delays have been caused at all airports cross Britain.

    British Airways (BA) on Thursday advised all passengers that they would not be permitted to carry any hand baggage on board any aircraft departing from any airport in Britain.

    Passengers will be allowed to take only travel documents, eyeglasses and a handful of other items through airport security, the company said in a statement.

    BA also advised passengers that no electrical or battery powered items including laptops and mobile phones could be allowed in the cabin.

    The London Heathrow airport is now closed to all incoming flights. BA has cancelled all short-haul European flights as well as severe delays to services. And Olympic Airways has grounded all flights.

    Meanwhile, the Downing Street said that Prime Minister Tony Blair, on holiday in the Caribbean, is in constant contact with the UK anti-terror team. The operation undertaken is with full support from the prime minister, it said.

    Blair also kept U.S. President George W. Bush informed on security threat to planes, the Downing Street added.

    The United States has raised threat level in response to the terror plot in Britain. Enditem 

    Related: Britain's Heathrow Airport closed to all incoming flights

Travellers queue at Gatwick international airport on the outskirts of London, following the introduction of heightened security measures August 10, 2006.(Xinhua/Reuters Photo)

    LONDON, Aug. 10 (Xinhua) -- The London Heathrow Airport is now closed to all incoming flights, after police announced early Thursday morning the foiling of a major terrorist plot to blow up British planes destined for the United States.

    According to Sky TV reports, some 20 flights from London, Glasgow and Manchester had been targeted. The explosives involve certain liquid chemical devices. Full story>>

    Major UK air terror plot thwarted

Armed police officers patrol Manchester Airport. British police have said they had foiled a plot to blow up planes to the United States in mid-air, putting the nation on its maximum terror alert and snarling air traffic around the world.(Xinhua/AFP Photo)

    LONDON, Aug. 10 (Xinhua) -- Police in London said on Thursday they have thwarted a major terrorist plot to blow up aircraft in mid-flight.

    Scotland Yard said that the aim of the plot was to detonate explosive devices on board the aircraft in hand luggage.

    The plot was likely to target flights from Britain to the United States, it said.   Full story>>

     US raises threat level amid foiled terrorist plot in UK

    WASHINGTON, Aug. 10 (Xinhua) -- The United States early on Thursday raised its security threat level to the highest level "Red" for commercial flights from Britain to the United States in response to an aborted major terror plot in Britain.

    A statement from the U.S. Department of Homeland Security said, "For that reason, the United States government has raised the nation's threat level to Severe, or Red, for commercial flights originating in the United Kingdom bound for the United States." Full story>>

    Germany cancels flights to Britain amid terrorist threat

    BERLIN, Aug. 10 (Xinhua) -- Germany has cancelled nearly all flights from Germany to Britain in the wake of reports early Thursday that British police have foiled a major terrorist plot to blow up British planes destined for the United States.

    Lufthansa, the biggest airline in Germany and the second biggest in Europe, cancelled all flights scheduled to depart for Britain before 2 p.m. local time (1300 GMT), the DPA agency in a report. Full story>>

    Spain cancels flights to London amid fear of terrorist attack

    MADRID, Aug. 10 (Xinhua) -- The Spanish airline Iberia said Thursday that it was canceling flights to London's Heathrow Airport until noon after British police announced early Thursday they had thwarted a terrorist plot to blow up aircraft in mid-flight.

    Iberia said it had canceled two flights from Madrid and one each from Barcelona and Bilbao, following a request made by British airports authorities in the wake of the discovery of the terrorist plot. Full story>>