Paul Stephenson, a deputy police commissioner at Scotland Yard, said, "We think this was an extraordinarily serious plot, and we are confident that we've prevented an attempt to commit mass murder on an unimaginable scale."
In the United States, federal officials raised the warning level for flights arriving from the United Kingdom to its highest level and a notch below for all other aviation. It is the first time the red alert level in the Homeland Security warning system has been declared. The agency defines red alert as designating a "severe risk of terrorist attacks."
Chertoff said that until further notice, all passengers are banned from bringing gels and liquids aboard domestic and foreign flights.
Airlines were adopting additional restrictions, leaving fliers confused about what they could carry aboard. Passengers were advised to check with the airlines on which they would fly.
Chertoff said the gel and liquid restrictions were ordered because investigators believe that the plotters planned to bring liquids on board, "each one of which would be benign, but mixed together could be used to create a bomb."
The liquids were to be disguised as beverages, he said, and the detonators made to look like "electronic devices or other common devices."
Britain banned all cell phones and portable music players from flights.
Federal officials, including Chertoff, have announced several arrests recently involving homeland security risks that ultimately were not as serious as first portrayed. Among them were the arrest in June of seven men in Miami, who Attorney General Alberto R. Gonzalez warned intended to wage a "full ground war" against the United States. Only later did details emerge that the men had no vehicles, no weapons, no money and no other means to achieve their alleged goals.
Yesterday, in contrast, Chertoff called the latest plot "a very sophisticated plan and operation" that was close to fruition.
"They had accumulated the capability necessary, and they were well on their way," he said at a televised news conference in Washington.
Like many other airports, Baltimore-Washington International Thurgood Marshall Airport was the scene of long lines and missed flights yesterday morning. But the backups were mostly confined to domestic flights in busy Concourse D, and even there the lines were reduced to manageable levels by about 10:30 a.m.
State transportation officials said they knew of no intelligence suggesting that flights into and out of BWI were terrorist targets. British Airways flies one plane into and out of Baltimore each day, and yesterday's flight was delayed but not canceled.
Most affected by yesterday's security crackdown was London's Heathrow Airport, the busiest airport in the world on its slowest days and downright chaotic even on its best days in August, when the airport handles about 190,000 passengers and more than 1,200 flights daily.
Yesterday, tens of thousands of people ready to board flights home never got off the ground.
Hundreds of flights were canceled, and televised pictures of a barely budging flood of passengers stuck in the airport's narrow concourses were enough to persuade others to try flying another day.
The delays came after British officials -- followed by their U.S. counterparts -- announced details of the planned attacks, which they said involved liquid explosives smuggled in carry-on luggage.
Law enforcement officials have said that the targets of the plot were American, Continental and United Air Lines flights, AP reported, but airline officials said they have not been told which carriers or flights were the intended targets. At airports across the country, long lines of passengers discarded items as they waited to go through newly rigorous searches.
The potential for attacks involving liquid explosives has long been known.
"You can see from the disruptions why no action was taken against that until it was deemed necessary," said Ayers, the Chatham House terrorism expert. "The question is what's practical? Where do you draw the line? Flights could be even safer if everybody was required to fly naked and with no luggage."
Authorities in London did not say how many aircraft had been identified for attack. Sky News, a cable station in Britain, put the number at six, while other reports said between three and 10. A counterterrorism official said the plot involved 10 flights. Chertoff spoke of "multiple explosions in multiple planes."
White House spokesman Tony Snow said yesterday that President Bush had been briefed extensively over the past few days, which he has spent vacationing on his ranch in Crawford, Texas, and had spoken twice to British Prime Minister Tony Blair, who is on vacation in the Caribbean.
Wire services and Sun reporter Michael Dresser contributed to this article.