WASHINGTON — The Bush administration posted an unprecedented code-red alert for passenger flights from Britain to the United States and banned liquids from all carry-on bags Thursday, clamping down quickly after British authorities disrupted a frightening terror plot.

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-Qaida plot."

Los Angeles Mayor Antonio Villaraigosa said as many as 10 flights had been targeted.

Other officials said the plotters had hoped to bring the planes down in a cascade of horror over the Atlantic Ocean, possibly within days. They said the plan involved the use of a peroxide-based solution, flammable when sparked by innocent-appearing small electronic devices. The officials spoke on condition of anonymity, citing the sensitive nature of the subject.

The targets included United, American and Continental Airlines flights from Britain to major U.S. destinations of New York, Los Angeles and Washington, D.C., officials said.

In brief remarks from Green Bay, Wis., President Bush said the events showed the nation "is at war with Islamic fascists."

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 makeup 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.

It was not clear how long the restrictions would remain in effect. One lawmaker, Rep. Pete Hoekstra, R-Mich., said the situation "eliminates the days of carry-on baggage."

The plot quickly became grist for the midterm election campaign.