UPDATED 8:08pm: The "main players" behind a plot to blow up multiple aircraft mid-air have been arrested in raids this morning, the home secretary has said.

But John Reid said the "critical" level of alert at airports countrywide, allowing no hand luggage on planes, is a precaution. "Had this plot been carried out, the loss of life to innocent civilians would have on an unprecedented scale.

"We think the main players are in custody, but we should always err on the side of caution," he added at a West Minster press conference. "You can never at this stage be certain."

A total of 24 people have been arrested overnight - mostly in London, but also in the Thames Valley and Birmingham.

Police believe the aim was to smuggle bombs hidden in hand luggage onto as many as nine planes flying from Britain to America.

Prime minister Tony Blair briefed American president George W. Bush overnight about the raids, Downing Street confirmed.

For months, the intelligence operation has involved an "unprecedented level of surveillance", said deputy assistant commissioner Peter Clarke, head of the Met's anti-terror branch.

"We have been looking at meetings, movements, travel, spending and the aspirations of a large group of people."

'Critical point'

Last night a "critical point" was reached with a decision "to take urgent action in order to disrupt what we believe was being planned", Mr Clarke added.

"As always in these cases, the safety of the public was our overriding concern."

The home secretary confirmed Cobra, the cabinet's emergency committee chaired by him, met three times overnight to discuss the terror alert.

"Move too early, and you may not know the full scope of who are involved and you may provoke those you don't know," Dr Reid said. "Don't move and you run the risk of terrible consequences."

According to the Met's anti-terror chief, the plot had "global dimensions".

The bombs were to be made in Britain and taken through local airports on transatlantic flights, Mr Clarke said. "The number, destination and timing of the flights that might be attacked remain the subject of investigation."

Today's raids were only the "culmination of one phase" of a major operation that will "last long into the future", he added.

Blair praises police effort

The prime minister, currently on holiday in the Caribbean, praised the "immense effort" made by police and security services. "I thank them for the great job they are doing in protecting our country."

Mr Blair added: "There has been an enormous amount of cooperation with the US authorities which has been of great value and underlines the threat we face and our determination to counter it."

Police urged the public to stay calm and be vigilant.

All Britons face a common threat, the home secretary said. "Terrorists will not distinguish between those of different religions, or men and women, combatants or civilians, or children or adults. In general, terrorists will massacre anyone who stands in their way."