British authorities today said they had thwarted a plot to simultaneously blow up several U.S.-bound airliners in flight, using bombs in carryon bags.

Terrorists had targeted United, American and Continental airlines, two U.S. counterterrorism officials said.

The United States responded by raising its threat warning to the highest level for commercial flights from Britain to the United States and also raised the alert level for flights within and to or from the United States.

A U.S. counterterrorism official said authorities believe as many as 50 people were involved in the overseas plot, which was unraveled last night. The plan "had a footprint to al-Qaida back to it," said the official, speaking on condition of anonymity.

The terrorists "were not yet sitting on an airplane," but were very close to traveling, the official said, calling the plot "the real deal."

A British police source told Reuters news service the plot was believed to involve "a liquid chemical device." As of today, travelers weren't being allowed to take any liquids, gels or lotions aboard flights.

Homeland Security Secretary Michael Chertoff said, "We believe that these arrests (in London) have significantly disrupted the threat, but we cannot be sure that the threat has been entirely eliminated or the plot completely thwarted." Chertoff said the threat level for flights from Britain to the United States has been raised to the highest "severe or red" level.

"To defend further against any remaining threat from this plot, we will also raise the threat level to high, or orange, for all commercial aviation operating in or destined for the United States," Chertoff said.

The threat level also was raised at Hawai'i airports last night.

State Civil Defense last night said all passengers will undergo additional security screening at the boarding gates, particularly to ensure that travelers do not take any liquids or gels, including beverages, aboard the aircraft.

The Transportation Security Administration urged passengers to allow extra time for the screening. Domestic and international travelers should be at the airport at least three hours before their flights and interisland passengers at least two hours.

Maj. Gen. Robert Lee, Hawai'i's adjutant general and director of Civil Defense, said travelers should adjust their schedules, but not cancel their trips.

A U.S. official said there have been no arrests in the United States connected to the plot.

Another official said that U.S. intelligence agencies, particularly the CIA, have been working closely with the British for months on the investigation.

Authorities have not yet arrested or detained all suspects who are believed to be involved in the plot, the official said, prompting the alert within the United States. U.S. authorities planned a news briefing early today.

In London, Britain's Home Secretary John Reid said the alleged plot was "significant" and that terrorists aimed to "bring down a number of aircraft through mid-flight explosions, causing a considerable loss of life."

Police arrested 21 people overnight in London, its suburbs and in Birimingham, and said that searches continued in a number of areas.

The national threat level in Britain was raised to critical, indicating the likelihood of an imminent terrorist attack.

Heathrow Airport in London was closed for most European flights, and several European carriers canceled flights to Britain, where airports were experiencing massive delays.

Britain's Department of Transport said carryon bags, electronic items and liquids were banned from all flights.