Sources said the overnight arrests of British-born men in London, the Thames Valley and Birmingham were part of an operation that had been going on for several months, police said. While Metropolitan Police Deputy Commissioner Paul Stephenson assured the public that the plot, which was "intended to be mass murder on an unimaginable scale," had been thwarted, they're not taking any chances: At 2 a.m. Thursday, the Joint Terrorism Analysis Center, which analyzes terror threats for the British government, raised the terror alert to "critical," which the U.K.'s MI5 security service says indicates "the likelihood of an imminent terrorist attack." London's Heathrow airport, the world's busiest international airport, is closed to incoming short-haul flights, while long-haul flights in both directions are seriously delayed, and the British Airports Authority has banned passengers in all U.K. airports from taking hand luggage on board. Such essentials as travel documents, wallets and medication are exempt, but they must be carried in a clear plastic bag. No electrical or battery-powered items are allowed. Baby formula can be taken onboard, after it's been tasted by an accompanying passenger.
All this comes just one day after Home Secretary John Reid said Britain was facing the most sustained period of serious threat since the end of World War II. According to Michael Clarke, professor of defense studies at Kings College in London, there are currently over 60 people coming to trial on terror-related charges in Britain. "MI5 say they are actively tracking 1,200 or so people, which is three times as many as they were tracking five years ago," says Clarke. "This operation would have been the result of long-term surveillance. We are told that terrorists operate in cells, independently of each other, but every time a plot is broken up, it's surprising how many of the suspects do talk to each other." But the surprising thing is the reaction. "The police have made it clear that they think they've intercepted a plot that's fairly extensive and not due to take place immediately," he says. "So you'd think the alert level would be lower, not higher. This would imply that they think there's a piece of this plot still out there. The critical alert level doesn't mean that they expect an attack to happen, but it means the security services will behave as if it's expected. As a precautionary measure."