LONDON, AP – British authorities said Thursday they had thwarted a terrorist plot to simultaneously blow up several aircraft heading to the U.S. using explosives smuggled in hand luggage, averting what police described as “mass murder on an unimaginable scale.”

Officials raised security to its highest level in Britain — suggesting a terrorist attack might be imminent — and banned hand-carried luggage on all trans-Atlantic flights. Huge crowds formed at security barriers at London’s Heathrow airport as officials searching for explosives barred nearly every form of liquid outside of baby formula.

The extreme measures at one of the major international aviation hubs sent ripple effects throughout the world. Heathrow airport was closed to most flights from Europe.

The U.S. government responded by raising its threat alert to its highest level for commercial flights from Britain to the United States amid fears the plot had not been completely crushed. The alert for all flights coming or going from the United States was also raised slightly.

In Washington, two U.S. counterterrorism officials said the terrorists had targeted United, American and Continental airlines. They spoke on condition of anonymity because of the sensitivity of the case.

Police arrested 21 people in London, its suburbs and in Birmingham as part of a major covert counterterrorism operation that had lasted several months, Deputy Commissioner Paul Stephenson said. Searches continued in a number of locations.

Police are confident they have disrupted a plot against aircraft which was “intended to be mass murder on an unimaginable scale,” he said.

The suspects were “homegrown,” though it was not immediately clear if they were all British citizens, said a police official who spoke on condition of anonymity because of the sensitivity of the case. Police were working closely with the South Asian community, the official said.

The official said the plotters intended to simultaneously target multiple planes bound for the United States.

Prime Minister Tony Blair, vacationing in the Caribbean, briefed President Bush on the situation overnight, Blair’s office said. There was no immediate public reaction from the White House. Bush is spending a few days at his ranch near Crawford, Texas.

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.”

“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,” said U.S. Homeland Security Secretary Michael Chertoff.

Chertoff added, however, there was no indication of current plots within the U.S. Numerous flights from U.S. cities to Britain were canceled Thursday morning.

A senior U.S. counterterrorism official said authorities believe dozens of people — possibly as many as 50 — were involved in the plot, which “had a footprint to al-Qaida back to it.” The official spoke on condition of anonymity because of the sensitivity of the situation.

The plan involved airline passengers hiding masked explosives in carry-on luggage, the official said. “They were not yet sitting on an airplane,” but were very close to traveling, the official said, calling the plot “the real deal.”

Passengers in Britain faced delays as tighter security was hastily enforced at the country’s airports and additional measures were put in place for all flights. Laptop computers, mobile phones, iPods, and remote controls were among the items banned from being carried on board.

Liquids, such as hair care products, were also barred on flights in both Britain and the U.S., raising the possibility that authorities were searching for a liquid explosive.

Huge lines formed at ticket counters and behind security barriers at Heathrow and other airports in Britain. Ed Lappen, 55, a businessman from Boston, who was traveling with his wife and daughter to Russia, found himself unable to travel further.

“We’re safe, we’re OK,” he said at Heathrow. “Now my daughter is going to get a shopping trip in London.”

Hannah Pillinger, 24, seemed less concerned by the announcement. “Eight hours without an iPod, that’s the most inconvenient thing,” she said, waiting at the Manchester airport.

Most European carriers canceled flights to Heathrow because of the massive delays created after authorities enforced strict new regulations banning most hand baggage.

Heathrow’s block on incoming traffic applied to flights of three hours or less, affecting most of the incoming traffic from Europe, an airport spokesman said on condition of anonymity in line with airport policy.

Officials at Frankfurt’s airport, Europe’s second-busiest, Schiphol in Amsterdam and Charles De Gaulle in Paris said Heathrow-bound planes could instead land at their airports if they needed to.

London’s Heathrow airport was the departure point for a devastating terrorist attack on a Pan Am airplane on Dec. 21, 1988. The blast over Lockerbie, Scotland, killed all 259 people aboard Pan Am Flight 103 and 11 people on the ground.

The explosive was hidden in a portable radio which was hidden in checked baggage.