A major terrorist plot to take bombs onto US-bound planes had "global dimensions", Scotland Yard has said.

Deputy assistant commissioner Peter Clarke, head of the anti-terrorist branch at the Metropolitan Police, said the arrests, now totalling 24, in London and the West Midlands came after several months of work and an "unprecedented level of surveillance".

"As is so often the case in these investigations, the alleged plot has global dimensions," he said.

It is thought that flights between Britain and America were the key target, with planned attempts to set off explosive devices while they were in the air.

Metropolitan Police deputy commissioner Paul Stephenson earlier said the security services were "confident that we have disrupted a plan by terrorists to cause untold death and destruction and to commit quite frankly mass murder".

The US homeland security secretary Michael Chertoff, who has been working with British authorities, said the plot was "sophisticated" and "international in scope".

"This operation is in some respects suggestive of an al Qaeda plot, but because the investigation is still under way we cannot form a definitive conclusion," he said.

Government response

As the arrests were carried out, government intelligence experts raised the official UK threat level from 'severe' to 'critical', which means "an attack is expected imminently and indicates an extremely high level of threat to the UK".

Number 10 said Tony Blair has been fully briefed, and has spoken to US president George W Bush about the situation.

The prime minister also paid tribute to the "immense" effort made by the police and security services in thwarting the planned terror attacks.

Speaking from his Caribbean holiday after being briefed on the latest developments, he said: "I would like to pay tribute to the immense effort made by the police and security services who for a long period of time have tracked this situation and been involved in an extraordinary amount of hard work.

"I thank them for the great job they are doing in protecting our country."

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

It was reported that the British and American leaders have held discussions about the alleged plot over recent weeks, including during Blair's recent trip to Washington DC.

John Reid chaired sessions of the government's emergency Cobra committee, late on Wednesday night and again on Thursday morning.

Threat

The home secretary said that if the plot been carried out "the loss of life to innocent civilians would have been on an unprecedented scale".

"While the police are confident that the main players have been accounted for, neither they nor the government are in any way complacent," Reid added.

"This is an on-going, complex operation and we all believe that we have taken the necessary precautionary measures to protect the public, both by the actions we have taken and by the maintenance of that threat level for the time being at the highest possible level.

"The police and the security services, together with government officials, are working around the clock in order to prevent any further terrorist activity."

And transport secretary Douglas Alexander said the government had "issued instructions to UK airports and the airlines that operate from them requiring new security measures to be put in place".

"We greatly value the co-operation that has been shown so far, both by industry and by passengers," he said.

"I recognise that these measures are already affecting many families and individuals seeking to travel at this busy time of year.

"I would assure you, however, that every effort is being made to address the operational challenges being confronted by our airlines and our airports.

"While this work is underway I would urge all passengers to show understanding in the inevitably difficult circumstances that they will face."

Alert

With Britain at the highest possible alert, Stephenson said the security agencies would "take whatever action is necessary to protect people both here in London and right the way through the United Kingdom".

"We are looking to the public to remain calm, patient and vigilant but we cannot stress too highly the severity that this plot represented," added the Met deputy commissioner.

"Put simply, this was intended to be mass murder on an unimaginable scale. This is not about communities, this is about criminals, this is about people who want to commit mass murder.

"This is not about anything to do with any particular community, this is about people who might masquerade within a community behind certain faiths, but this is about people who are desperate people who want to do things that no right-minded citizen of this country or any other country would want to tolerate."

Both police officers and ministers have called on the public to remain vigilant.

In the US, Chertoff said the US government had raised the nation's threat level to its highest - severe or red - for commercial flights inbound from the US, and was raising the threat level on all other aviation to high, or orange.

Reaction

Shadow foreign secretary William Hague said that Conservative leader David Cameron had been briefed by the home secretary this morning, and said: "We're united today in facing a serious threat to the lives of very large numbers of people".

Mayor of London Ken Livingstone said: "We can only defeat terrorism by having the best possible intelligence and the co-operation of all Londoners .

"No community in London can or should be targeted or blamed because of the actions of people who are pure criminals.

"The efforts of the Metropolitan Police to maintain contact with leaders of all communities this morning are to be applauded."