Mothers tasted baby food in front of airport security guards to prove it contained no liquid explosives. Liquids and gels were banned from flights. Travelers repacked their luggage in airports, stowing all but the most necessary items in the hold.

Although plots to blow up airliners using liquid explosives are not new — such an attempt was foiled more than a decade ago — the U.S. government has been slow to upgrade its security equipment at airport checkpoints to detect explosives on passengers.

U.S. authorities did not say how long the security measures would last. "We are taking the step of preventing liquids from getting into the cabin to give us time to make adjustments," Chertoff said.

The raids in Britain on Thursday followed a monthslong investigation, but U.S. intelligence officials said authorities moved quickly after learning the plotters hoped to stage a practice run within two days, with the actual attack expected just days after that.

The test run was designed to see whether the plotters would be able to smuggle the needed materials aboard the planes, the officials said, speaking on condition of anonymity. Security experts warned it was possible attackers could hide their bomb ingredients in containers for talcum powder or medicine bottles and then assemble the weapon once behind a locked restroom door.

U.S. counterterrorism officials said United, American, and Continental airlines bound for New York, Washington and California were targeted by the terrorists. Los Angeles Mayor Antonio Villaraigosa said the individuals plotted to detonate liquid explosive devices on as many as 10 aircraft bound for the United States.

"This nation is at war with Islamic fascists who will use any means to destroy those of us who love freedom, to hurt our nation," President Bush declared.

The plane bombings could have come just ahead of the fifth anniversary of the Sept. 11 attacks carried out by al-Qaida. The terror group's leader Osama bin Laden and his No. 2, Ayman al-Zawahri, are believed hiding along the Pakistan-Afghanistan border and have repeatedly issued tapes threatening new attacks.

"In terms of scale, it was probably designed to be ... a new Sept. 11," said Jean-Charles Brisard, a French private investigator who works with lawyers of many Sept. 11 victims. "It involved the same tools, the same transportation tools and devices."

The close call also shifted attention once more to Britain's Islamic community just over a year after the London transit attacks. Three Britons of Pakistani descent and a Jamaican convert to Islam carried out those deadly bombings with a peroxide-based explosive that trained operatives can make using ordinary ingredients such as hair bleach.