LONDON – British authorities said Thursday that they had thwarted a terrorist plot to blow up multiple airliners traveling between Britain and the United States, creating “mass murder on an unimaginable scale.”
Passengers thronged London’s Heathrow Airport on Thursday after the announcement that British police had thwarted a terrorist plot involving flights originating in the United Kingdom.
The police said they had arrested 21 people in connection with the plot, which apparently involved plans to smuggle explosives onto aircraft in hand luggage. In response, flights into London Heathrow Airport were canceled and airlines banned hand luggage on departing planes, causing chaos and long delays.
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.
“We think this was an extraordinarily serious plot and we are confident that we have stopped an attempt to create mass murder on an unimaginable scale,” Stephenson told reporters at Scotland Yard.
He said the people had been arrested in and near London and Birmingham, and added that the searches would continue.
“We have been very successful in arresting those we were targeting but this is a lengthy operation, and no doubt there will be further developments,” he told reporters.
News of the foiled plot comes little more than a year after terrorists killed 52 people in an attack on the London Underground, and nearly five years after the attacks of Sept. 11.
As heightened security measures went into effect at Heathrow, travelers were sometimes given little explanation, amid chaotic scenes.
Joanne Weslund, 68, a retired schoolteacher from Hubbardston, Mass., was critical of the way the situation had been handled by the airlines.
“It’s been terrible,” she said. “We are waiting in Disney-like lines. The only thing B.A. has said is it’s a security breach. We are told we can bring nothing on the plane, only passport and cash. If there is a threat, people should not be on planes, but how they handled this is atrocious.”
A customer service agent for British Airways told passengers, “The only thing we know for sure is Christmas Day falls on Dec. 25.”
OAG, the transport industry information company, estimated that 400,000 people in Britain would be affected by the security alert. On a typical day, the firm said, 3,800 flights take off from Britain, though only about 3 percent of those are trans-Atlantic.
Officials were requiring passengers to check everything except personal items like keys, wallets, and passports, which they had to carry in plastic bags. Drinks and other liquid items were banned.
Travelers were required to remove spectacles or sunglasses from their cases, and those travelling with infants were required to taste any baby milk in front of security officials.
Britain’s Department for Transport said it was requiring secondary searches of travelers headed for the United States, with a particular eye to removing any liquids they might have with them.
“We hope that these measures, which are being kept under review by the government, will need to be in place for a limited period only,” the department said in a statement.
Despite the arrests, the police said other people connected with the plot might still be at large.
“We believe that these arrests have significantly disrupted the threat, but we cannot be sure that the threat has been entirely eliminated or the plot completely thwarted,” Michael Chertoff, the U.S. homeland security secretary, told reporters.
The anti-terrorist action came at a time when Prime Minister Tony Blair is on vacation in the Caribbean. The government said he has spoken to President George W. Bush about the situation.
Disclosure of the plot came one day after John Reid, the British home secretary, gave a speech in which he warned that Britain faced “probably the most sustained period of severe threat since the end of the Second World War.”
“This has involved close cooperation, not only between agencies and police forces in the United Kingdom, but also internationally,” Reid said.
The British government has come under criticism for anti-terrorism measures imposed in the wake of the July 7 bombings last year.
Despite the extra security measures, flights were still taking off from Heathrow on Thursday morning. Other British airports also reportedly faced delays and crowding.
Ann Keating, a private investigator from Salem, Mass., who landed at Heathrow on Thursday morning, said that because of the chaotic conditions, “the ones I fell most sorry for are the kids.
Karla Adams contributed to this article.