"It was sophisticated, it had a lot of members and it was international in scope," said Chertoff. "It was in some respects suggestive of an al-Qaida plot."

He added, however, that "because the investigation is still under way we cannot yet form a definitive conclusion."

Gonzales said the operation could "potentially kill hundreds of innocent people." Britain said 21 people had been arrested, including the alleged "main players" in the plot.

Mueller also pointed at al-Qaida. "This had the earmarks of an al-Qaida plot," he said.

The alleged plot was "as sophisticated as any we have seen in recent years as far as terrorism is concerned," Chertoff said.

He said there was no indication of any plotting in the United States but that the government was taking steps to protect against unseen threats or copycat attacks. "We cannot assume that this threat has been completely thwarted," the secretary said.

"There's sufficient uncertainty as to whether the British have scooped up everybody," Chertoff added. Gonzales said the operation could "potentially kill hundreds of innocent people." Chertoff said the plot was "as sophisticated as any we have seen in recent years as far as terrorism is concerned."

Hastily printed signs were posted at major airports warning passengers in red capital letters, "No liquid or gels permitted beyond security."

It is the first time the red alert level in the Homeland Security warning system has been invoked, although there have been brief periods in the past when the orange level was applied. Homeland Security defines the red alert as designating a "severe risk of terrorist attacks."

There were no commercial passenger planes in the air from Britain to the United States when the red alert was issued, FAA Administrator Marion Blakey said. She said three cargo planes aloft from London - two Lufthansa and one UPS plane - were allowed to continue because the threat was focused on passenger planes.

Officials said the government has been aware of the nature of the threat for several days, and

President Bush, vacationing in Texas, was fully briefed. Prime Minister Tony Blair's office said in London that the prime minister, vacationing in the Caribbean, had briefed Bush overnight.

The U.S. Northern Command, the military headquarters established in response to the Sept. 11, 2001 attacks, was "monitoring and ... a little bit more vigilant today," said spokesman Michael Kucharek, declining to be more specific.

"I'm not going to say it's business as usual," he said. "We're looking at all sources of information - this is a real threat to the nation."

The plot was not believed to be connected to a group of Egyptian students who disappeared in the United States more than a week ago before reaching a college they were supposed to attend in Montana. Three of the 11 have since been found and the FBI has said neither they nor the still-missing eight are believed to be a threat.

As part of the foiled Bojinka Plot to blow up 12 Western airliners simultaneously over the Pacific Ocean in the mid-1990s, terrorist mastermind Ramzi Youssef planned to put together an improvised bomb using liquid in a contact lens solution container.

The metal detector and X-ray machines at airport security checkpoints cannot detect such explosives. At many, but not all airport checkpoints, the TSA has deployed walkthrough "sniffer" or "puffer" machines that can detect explosives residue.

American Airlines parent is AMR Corp.

United's parent is UAL Corp.

Continental's full name is: Continental Airlines Inc.