Deputy Police Commissioner Paul Stephenson said Thursday that police and security services intercepted what they believe was a plot to blow up several U.S.-bound aircraft.

"We believe that the terrorists' aim was to smuggle explosives onto airplanes in hand luggage and to detonate these in flight," Stephenson says. "We also believe that the intended targets were flights from the United Kingdom to the United States of America."

Reports say up to 10 airplanes were targeted.  Exactly how far along the plot had actually advanced was not revealed.  But Commissioner Stephenson said the plot was a serious one.

"We cannot stress too highly the severity that this plot represented," Stephenson says. "Put simply, this was intended to be mass murder on an unimaginable scale."

He said police initially detained 21 people in connection with the plot, and that the investigation is continuing.  Most were arrested in London, but raids were also carried out also in the capital's suburbs and in the city of Birmingham.

The security threat level was raised to "critical," the highest such level in Britain.

All flights coming into Britain were canceled, and many outgoing flights were either delayed or canceled altogether.

Stringent new security measures were immediately imposed at all British airports, with particular attention paid to U.S.-bound flights.  Transport Secretary Doug Alexander said virtually all hand luggage is now banned from airplane cabins.

"What these changes mean in practice is that all hand baggage will now have to be checked in, with only a small number of essential items allowed through search controls," Alexander. "Exceptions will be place for those traveling with infants, and for prescription medicines.  Extra security measures will be in place for flights to the United States of America."

Tony Douglas, managing director of the British Airports Authority, said Heathrow, Britain's busiest airport, had been particularly affected by the new security measures, and advised people not to travel unless absolutely necessary.

"As a result, BAA Heathrow is experiencing severe disruption to its operation.  Check-in and hand search processes across all four terminals is severely affected, and this will continue throughout the day," Douglas says.

Transport Secretary Alexander did not say if or when the stringent security would be loosened, saying the measures will remain in place, as he put it, "for as long the situation demands.