Officials raised security to its highest level - suggesting a terrorist attack may imminent-and banned handcarried luggage on all trans-Atlantic flights. Huge crowds formed at security barriers.

The extreme measures at one of the major international aviation hub sent ripple effects throughout the world. Officials at Heathrow airport cancelled most flights from Europe.

The U.S. government responded by raising its threat assessment to its highest level for commercial flights from Britain to the United States amid fears the plot had not been completely crushed.

Police are confident they have disrupted a plot against aircraft which was "intended to be mass murder on an unimaginable scale," Deputy Police Commissioner Paul Stephenson said Thursday.

Stephenson said 21 people had been arrested in London, its suburbs and in Birmingham, and that searches continued in a number of locations.

Prime Minister Tony Blair, vacationing in the Caribbean, briefed U.S. President George W. Bush on the situation overnight, Blair's office 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."

"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.

Passengers faced delays as tighter security was hastily enforced at the country's airports and additional measures were put in place for all flights, and British Airways said some flights were likely to be cancelled. Laptop computers, mobile phones, IPods, and remote controls were among the items banned from being carried on board...

...The police did not identify the suspects or their origin, though Paul Stephenson, the deputy metropolitan police commissioner for London, said "community leaders" had been alerted about the police action, using a code word for the British Muslim community.

The authorities did not say how many aircraft had been identified for attack. Sky News put the number at six, while other reports said between three and 10...

ALMOST a quarter of British Muslims say the 7/7 bombings can be justified because of the Government's support for the war on terror, according to an opinion poll.

And nearly half of those polled, or 45 per cent, believe the 9/11 attacks on New York were a conspiracy between the United States and Israel. The survey, for a Channel 4 Dispatches documentary to be screened tonight, found Muslims under 24 were twice as likely to justify the 7/7 attacks as those aged over 45. It found 24 per cent either agreed or tended to agree that the 7/7 bombings were justified, although 48 per cent said they "strongly disagreed".

A third of those questioned said they would rather live under Sharia law in the UK than British law...