WASHINGTON (CNN) -- British and Pakistani authorities joined forces to block a plot to bomb U.S.-bound airliners, officials said Thursday.

British police acted urgently overnight, arresting 21 people who were plotting to carry liquid explosives onto as many as 10 jetliners in what U.S. government officials said privately could have been the biggest terrorist attack since 9/11.

Information gathered after recent arrests in Pakistan convinced British investigators they had to act urgently to stop the plot, sources told CNN.

A senior congressional source said it is believed the plotters, operating in teams, were to mix a "British version of Gatorade" with a gel-like substance. The source said they may have planned to use an MP3 player or a cell phone to trigger the explosives.

U.S. Homeland Security Secretary Michael Chertoff said the plans were "suggestive of an al Qaeda plot," and President Bush said the arrests are a "stark reminder" that the U.S. is "at war with Islamic fascists." (Watch Bush say what the arrests mean for the U.S. -- 2:37)

Bush thanked British Prime Minister Tony Blair for "busting this plot." (Full story)

Authorities immediately banned all passengers headed to or departing from U.S. airports from carrying any liquid in their carry-ons. The massive lines that resulted at security checkpoints made chaos of air travel worldwide as flights were delayed or canceled. (Full story)

The effects of the plot rippled across the globe Thursday.

The U.S. raised the terror threat level to "severe" for all flights leaving Britain for the United States. Britain raised its alert level to "critical." Massachusetts Gov. Mitt Romney ordered the National Guard to Boston's Logan Airport, and California Gov. Arnold Schwarzenegger ordered the National Guard to airports in his state. Besides banning liquids, British police are also banning passengers from carrying electronic key fobs, which have the potential to trigger bombs. (Watch what passengers must do at the world's busiest international airport -- 3:06) A U.S. administration official said the plot targeted Continental, United, British Airways and American Airlines flights to New York, Washington and California. (Map of flight delays) As many as 10 flights were targeted, and the plot may have involved up to 50 people, U.S. officials told CNN. U.S. and British officials said some suspects could still be on the loose and their investigations were continuing. Indications are that all of the 21 arrested were British citizens and some were of Pakistani ethnicity, a senior U.S. intelligence official said. In a sign of the heightened security, Chertoff said the U.S. was dispatching extra air marshals to Britain. Chertoff said it was unclear if all suspects were in custody. Chertoff said the plotters were "getting close to the execution phase." "There were very concrete steps under way to execute all elements of the plan," he said. The plot was "intended to be mass murder on an unimaginable scale," London's Metropolitan Police Deputy Commissioner Paul Stephenson said. (Full story) Chertoff said the plan was reminiscent of a plot by 9/11 coordinator Khalid Shaikh Mohammed, who in 1995 had envisioned detonating bombs on 11 airlines possibly traveling over the Pacific Ocean. The plot was "as sophisticated as any we have seen in recent years as far as terrorism is concerned," Chertoff said. (Watch Chertoff detail the threat to blow up jetliners -- 7:38) CNN terror analyst Peter Bergen said two factors pointed to the influence of al Qaeda. He said al Qaeda was "obsessed" with commercial aviation and that the timing of the plot was "very interesting." "It's not clear when this was going to be implemented ... but we are coming up on the fifth anniversary of 9/11. They do want to make a big statement," he said on CNN's "American Morning." The U.S. threat level has been raised to the highest level of "severe," or red, for commercial flights originating in the United Kingdom bound for the United States, according to the Department of Homeland Security. (Full story) In addition, the threat level has been raised to "high," or orange for all commercial flights operating in or coming to the United States, the DHS said. The nation's overall terror threat level has not been altered. Thursday was the first time the DHS has raised the threat level for a specific group of flights. New security restrictions "Due to the nature of the threat revealed by this investigation, we are prohibiting any liquids, including beverages, hair gels, and lotions from being carried on the airplane," a DHS statement said. Increased security means airline passengers around the country should show up at least two hours early for all flights, an official with the Transportation Security Administration told CNN. (Watch tips on how to get through security faster -- 1:45) British and U.S. security agencies quickly moved to impose strict limits on carry-on items in the wake of Thursday's arrests, causing extended delays at airport security checkpoints. (Full story) The British Airports Authority said no hand luggage would be allowed onto planes leaving British airports until further notice. British Airways canceled all short haul flights in or out of Heathrow Airport for Thursday, and delays were stacking flights up at airports across Europe.