LONDON - British authorities thwarted a terrorist plot to blow up several aircraft mid-flight between the United States and Britain using explosives smuggled in hand luggage, officials said Thursday.

"A major terrorist plot to allegedly blow up aircraft in mid-flight has been disrupted in a joint, pre-planned, intelligence-led operation by the metropolitan police anti-terrorist branch and security services," a police spokesman said.

Britain’s Home Secretary John Reid said the alleged plot was “significant” and that terrorists aimed to “bring down a number of aircraft through mid-flight explosions, causing a considerable loss of life.”

Police arrested a number of people overnight in London after a major covert counterterrorism operation that had lasted several months, but did not immediately say how many.

The national threat level was raised to critical — a warning level that indicates the likelihood of an imminent terrorist attack. The threat rating was posted on the Web site of Britain’s MI5 — the British domestic spy agency.

The U.S. government raised its threat level to the highest level for commercial flights from Britain to the United States early Thursday.

“We believe that these arrests (in London) have significantly disrupted the threat, but we cannot be sure that the threat has been entirely eliminated or the plot completely thwarted,” said U.S. Homeland Security Secretary Michael Chertoff.

Prime Minister Tony Blair, vacationing in the Caribbean, had briefed President Bush on the situation overnight, Blair’s office said.

The Department of Transport advised all passengers that they would not be permitted to carry any hand baggage on board any aircraft departing from any airport in the country.

Passengers faced delays as tighter security was hastily enforced at the country’s airports and additional measures were put in place for all flights. British Airways said laptop computers, mobile phones, iPods and remote controls were among the items banned from being carried on board.

those in custody are all Paki "citizens" from what they are saying.

http://www.msnbc.msn.com/id/14278216/