Plan Was to Sneak Liquid Explosives on Planes

LONDON, Aug. 10 — The British authorities said Thursday that they had thwarted an advanced terrorist plot to blow up airplanes flying from Britain to the United States using liquid explosives that would have escaped airport security.

The officials said they had arrested 24 men, all British-born Muslims, who planned to carry the liquids in drink bottles and combine them into explosive cocktails to commit mass murder aboard as many as 10 flights high over the Atlantic.

Intelligence officials said they believed that some plotters were probably still at large, requiring increased airport security.

Airports, which faced chaotic delays and cancellations, instantly changed rules on what passengers could carry on board. In the United States, liquids, gels and creams were banned from carry-on luggage. In Britain, all carry-on items were barred except objects like wallets and eyeglasses without their cases.

Officials said the plot — of which few concrete details were made known — bore the hallmarks of Al Qaeda and involved links to plotters in Pakistan.

Late Thursday, the authorities in Pakistan said an unspecified number of arrests had been made there, too.

An American counterterrorism official, who spoke in return for anonymity because of the sensitivity of the case, said several of the plotters had traveled to Pakistan in the last few weeks and might have met there with at least one person affiliated with Al Qaeda. The official said it was after that person’s arrest by Pakistani authorities that the British, fearing that word of the detainment would send the plotters into hiding, decided to move in.

This is the latest in a series of conspiracies apparently rooted in the disaffection of young, British-born Muslims, many of Pakistani descent, who cast themselves as part of a jihadist struggle against Britain, which they see as an outrider of the United States in Iraq, Afghanistan and now Lebanon.

It also mimicked a failed plot in the Philippines in 1995 financed by Osama bin Laden to blow up airplanes over the Pacific. That ended when the chemicals exploded at an apartment in Manila.

On Thursday, Britain raised its terror threat assessment by one notch to its highest level, “critical,” meaning an attack was imminent.

The American official said the plotters were planning a “dry run” of the operation in the next few days when they planned to test whether they could board flights simultaneously. If this had worked, a full-scale attack would have been carried out within days, the official said.