The heightened restrictions triggered long lines at airports across the country, and governors in at least three states ordered National Guard troops to help provide security.
"This was a well-advanced plan," Homeland Security Secretary Michael Chertoff told reporters as British authorities announced the arrests of 24 alleged plotters. "In some respects suggestive of an al-Qaeda plot."
Los Angeles Mayor Antonio Villaraigosa said as many as 10 flights had been targeted, and other officials said the plotters had hoped to bring the planes down in a cascade of horror over the Atlantic Ocean, possibly within days.
The plan involved the use of a peroxide-based solution, flammable when sparked by innocent-appearing small electronic devices.
The targets included United, American and Continental Airlines flights from Britain to major US destinations of New York, Los Angeles and Washington, D.C, officials said.
Virginia's deputy homeland security director, Steven Mondul, said that in a conference call, federal officials pointed to New York's John F. Kennedy Airport, Los Angeles International and Dulles Airport outside Washington as "major destinations for flights originating from the United Kingdom." No specific warnings were issued for these facilities, he added.
The red alert for flights from Britain was the first since the color-coded warning system was developed after the 2001 terror attacks. The decision to ban nearly all liquids from passenger cabins was reminiscent of the stringent rules imposed when planes were allowed back in the skies for the first time afterward the Sept. 11 attacks.
"No liquids or gels will be allowed in carry-on baggage," Chertoff said. "There will be exceptions for baby formula and medicines, but travelers must be prepared to present these items for inspection at the checkpoint, and that will allow us to take a look at them and make sure that they're safe to fly."
That meant water containers, soft drinks, coffee cups and more had to be shed by passengers waiting to board their flights.
Women travelers surrendered bottles and jars of creams and lotions from their make-up kits.
At Dulles, one passenger fished a bottle of Tequila from a carry-on bag. It joined the rest of the newly classified contraband in a trash container.
The plot quickly became grist for the midterm election campaign.
House Democratic Leader Nancy Pelosi said events reinforced the need to implement the recommendations of an independent 9/11 commission, a reminder of one of her party's main campaign promises.
In Ohio, Republican chairman Bob Bennett accused the Democrats' senatorial challenger of voting against funds "for the very types of programs that helped the British thwart these vicious attacks."
Homeland Security deputy Secretary Michael Jackson said his agency had known for several days of the unfolding plot but waited for a signal from the British to announce it.
'This was a very close call'
The decision to raise the terror level for flights from Britain indicated a severe risk of terror attacks. The "code red" change requires airlines to provide the government with an advance list of passengers aboard affected flights. Previously, passengers’ names had to be provided within 15 minutes after take-off.
All other flights to and within the United States were put under an "orange" alert, one step below red, but an escalation from the "yellow" status that had been in effect.
Administration officials sought to reassure the traveling public at the same time they imposed heightened security restrictions.
"Today, air traffic is safe, and air traffic will remain safe precisely because of the measures we are adopting today," Chertoff said.
Gov. Arnold Schwarzenegger of California deployed 300 Guard troops to at least three large airports _ in Los Angeles, San Francisco and Oakland _ where direct flights from Europe were scheduled to arrive.
In Massachusetts, Gov. Mitt Romney announced he would activate Guard troops for airline security duty for the first time since the terror attacks of 2001.
White House spokesman Tony Snow said President Bush had been briefed in advance of the events and had approved raising the alert to red on flights from England.
In brief remarks from Green Bay, Wis., the president said the events showed the nation "is at war with Islamic fascists."
Senior lawmakers also had received advance word. Several said they had been briefed by Homeland Security or CIA officials as early as Monday.
Officials said the plotters had been planning a test run within two days to see whether they could smuggle the equipment they needed aboard the flights. The actual attack would have occurred within days.
Chertoff, interviewed on CNN, did not challenge the account.
"It's not uncommon that these kinds of plots almost always have a dry run or a casing element before the actual plot is carried out," he said.
The attacks of Sept. 11, 2001, "had dry runs as well," he said.
Rep. John Kline, R-Minn., was the only lawmaker to attend a closed-door briefing in the Capitol.
"This was a very close call," he said of the aborted plot.