British police arrested 24 people in overnight raids and said on Thursday they had foiled a plot to commit "mass murder" by blowing up several aircraft flying over the Atlantic Ocean to the United States.

Tough new security measures brought chaos to airports on both sides of the Atlantic in what President Bush described as a stark reminder his country was at "war with Islamic fascists".

Pakistan said it intelligence agencies helped thwart the plot and had arrested an unspecified number of people.

"We are confident we have disrupted a plan by terrorists to cause untold death and destruction," said London police's Deputy Commissioner Paul Stephenson. "Put simply, this was intended to be mass murder on an unimaginable scale."

A U.S. Homeland Security department official said all the people arrested in Britain were British citizens. Overnight raids were carried out in the capital London, southeast England and Britain's second city, Birmingham.

The suspected plot raised the specter of a strike to rival the scope of the September 11, 2001, attacks on the United States ahead of their fifth anniversary, and came 13 months after four British suicide bombers killed 52 commuters on London transport.

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