British police are questioning 24 suspects after authorities said they foiled a terror plot to simultaneously blow up several aircraft in mid-flight between Britain and the United States, potentially killing thousands of people.

The plan was to smuggle liquid explosives in hand luggage on up to 10 planes, officials said Thursday.

Police say they are convinced the "major players" involved in the plot are in custody, but a wider investigation is only just beginning.

ABC News reported that authorities were urgently hunting five more suspects, quoting unidentified sources who had been briefed on the plot.

A U.S. law-enforcement official in Washington said at least one martyrdom tape was found during continuing raids across England on Thursday, reported The Associated Press. Such a tape, as well as the alleged plan to strike a range of targets simultaneously, is an earmark of al Qaeda.

British Home Secretary John Reid said had the bomb plot been carried out, "the loss of life to civilians would have been on an unprecedented scale."

The 24 suspects were arrested in London, High Wycombe and Birmingham and police searches were continuing in a number of homes.

There were also reportedly arrests in Pakistan in connection with the plot.

"There were some arrests in Pakistan which were co-ordinated with arrests in the U.K.," the country's Foreign Ministry spokeswoman Tasnim Aslam told The Associated Press.

She did not give the news agency specific details about the arrests. But another government official, speaking on condition of anonymity, said "two or three local people" were arrested earlier this week in Lahore and Karachi.

Metropolitan Police Deputy Assistant Commissioner Paul Stephenson said the alleged plotters had intended "mass murder on an unimaginable scale".

"We are confident that we have disrupted a plan by terrorists to cause untold death and destruction and to commit, quite frankly, mass murder," he told reporters.

It's believed three U.S. airlines -- United, American and Continental -- were to be targeted on flights to major airports in New York, Washington and California.

Reports say U.S intelligence officials believe a terror attack was imminent -- that plotters had hoped to stage a test run within two days, with an actual attack expected just days after that.

Suggestive of al Qaeda plot

The plane bombings could have arrived just ahead of the fifth anniversary of the Sept. 11, 2001 attacks carried out by al Qaeda.

"This nation is at war with Islamic fascists who will use any means to destroy those of us who love freedom, to hurt our nation," U.S. President George Bush declared.

U.S. Homeland Security Secretary Michael Chertoff said the sophistication and scope of the threat was "suggestive of an al Qaeda plot."

The plot involved hiding liquid explosives disguised as beverages or other common objects in carry-on luggage, Chertoff told reporters.

American law enforcement officials, who spoke on condition of anonymity, said the peroxide-based solution would be detonated by a power supply from a camera, a digital music player or other electronic device.

London's Heathrow Airport was closed on Thursday to many incoming flights not already in the air, while security was increased at all British airports.

Security was raised to its highest level in Britain and carry-on luggage on all trans-Atlantic flights was banned.

Passengers were only permitted to take aboard essential items, such as passports and wallets, and only then in a transparent plastic bag.

Meanwhile, massive crowds formed at security barriers at London's Heathrow Airport as officials searching for explosives barred just about every type of liquid except baby formula.

CTV's Bill Kearney, who was awaiting to board a flight from London to Ottawa, said it was a "crazy" scene at Heathrow Airport.

"It's very congested but they are moving passengers through," he told Newsnet Thursday. "Heavily armed police are walking around and people are panicking because they don't know what to do with their hand luggage."

Counter-terrorism investigation

The counter-terrorism investigation has been ongoing for several months, Metropolitan Police anti-terrorist branch chief Peter Clarke said.

Clarke said the investigation had "global dimensions" and had seen an "unprecedented level" of surveillance.

"We've been looking at meetings, movements, travel, spending and the aspirations of a large group of people," Clarke told reporters.

The decision to take action had been taken on Wednesday night, when the plot reached "a critical level," he added.

Police haven't released any information yet as to whether the suspects have ties to any particular group, although it's believed the suspects were "homegrown" and mostly British citizens. One U.S. law-enforcement official told AP at least some of the suspects were of Pakistani ancestry.

Britain's MI5 has raised the country's security threat level to "critical," meaning a terrorist attack "is expected imminently and indicates an extremely high level of threat," MI5's website says.

The threat level had been described as "severe" prior to Thursday's announcement.

Meanwhile, the U.S. Department of Homeland Security increased the threat level applied to U.S.-bound commercial flights originating in Britain to "red," its highest level.

A Downing Street spokesperson said British Prime Minister Tony Blair, who is vacationing in the Caribbean, had briefed Bush on the situation overnight.

Blair later paid tribute to police and security services, saying they had tracked the situation for a "long period of time" and had "been involved in an extraordinary amount" of hard work.

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

The alleged terror plot comes almost five years after Briton Richard Reid attempted to detonate explosives hidden in his shoes on an American Airlines flight from Paris to Miami in December 2001, just three months after the September 11, 2001 attacks in New York and Washington.

Reid pleaded guilty to terrorism charges in October 2002 and is serving a life sentence at the nation's super-maximum security prison in Florence, Colo.