British authorities said Thursday they had thwarted a terrorist plot to simultaneously blow up several aircraft flying to the United States using explosives smuggled in hand luggage, averting what police described as "mass murder on an unimaginable scale."
Police arrested 21 people, and said they were confident they had captured the main suspects in the alleged plot.
Officials raised security to its highest level - suggesting a terrorist attack might be imminent - and banned hand-carried luggage on all trans-Atlantic flights. Huge crowds formed at security barriers as officials searching for explosives barred nearly every form of liquid outside of baby formula.
"This was to be a simultaneous attack on multiple targets, targeting U.S.-bound aircraft," a police official said, speaking on condition of anonymity because of the sensitivity of the case.
The extreme measures at one of the world's largest aviation hubs sent ripples throughout the world. Heathrow airport was closed to most flights from Europe.
The U.S. government responded by raising its threat assessment to its highest level for commercial flights from Britain to the United States amid fears the plot had not been completely crushed. Terrorists had targeted United, American and Continental airlines, two U.S. counterterrorism officials said.
Police are confident they have disrupted the plot against aircraft, which was "intended to be mass murder on an unimaginable scale," Deputy Police Commissioner Paul Stephenson said Thursday.
Police arrested 21 people in London, its suburbs and in Birmingham, central England. Searches continued in a number of locations.
The suspects were "homegrown," though it was not immediately clear if they were all British citizens, the police official said, adding that police were working with the South Asian community.
Prime Minister Tony Blair, vacationing in the Caribbean, briefed U.S. President George W. Bush on the situation, Blair's office said.
The months-long investigation into the suspected terrorist plot had global dimensions and involved unprecedented surveillance, police anti-terrorist chief Peter Clarke said.
Britain's Home Secretary John Reid said Britain's threat status would remain at the highest level for the time being "as a precautionary measure."
"Whilst the police are confident that the main players have been accounted for, neither they nor the government are in any way complacent," Reid said. "This is an ongoing, complex operation."
Passengers faced delays as tighter security was hastily enforced at the country's airports and additional measures were put in place for all flights. British Airways canceled all flights Thursday between Heathrow airport and points in Britain, Europe and Libya.
Laptop computers, mobile phones, iPods and remote controls were among items banned from being carried on board.
Liquids, such as hair care products, were also barred. U.S. Homeland Security Secretary Michael raised the possibility that authorities were searching for a liquid explosive.
"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," Chertoff said.
Authorities in the United States imposed bans on carryon aerosols and most liquids on planes serving East Asia in 1995, after a plot was uncovered to use liquid bombs to bring down U.S. aircraft over the Pacific.
The plot was allegedly developed by Ramzi Yousef, alleged mastermind of the 1993 World Trade Center bombing, and was discovered after police raided his apartment in Manila in 1995. Police believed Yousef was in the Philippines to try to assassinate Pope John Paul II during a visit to Manila in 1995.
Huge lines formed at ticket counters and behind security barriers at Heathrow and other airports in Britain. Ed Lappen, 55, a businessman from Boston traveling with his wife and daughter to Russia, found himself temporarily stranded.
"We're safe, we're OK," he said at Heathrow. "Now my daughter is going to get a shopping trip in London."
The Department of Transport advised all passengers that they would not be permitted to carry any hand baggage, electrical items or battery powered items on board any aircraft departing from the country.
Prescription medicines were OK; so were eyeglasses but not their cases, the department said. Contact lenses could be taken aboard in their cases, but bottles of solution were banned.
"Eight hours without an iPod, that's the most inconvenient thing," said Hannah Pillinger, 24, who was waiting at the Manchester airport.
London's Heathrow airport was the departure point for a devastating terrorist attack on a Pan Am Boeing 747 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 in checked baggage.
A Scottish court convicted Libyan intelligence agent Abdel Basset Ali al-Megrahi of the bombing in 2001 and sentenced him to life imprisonment. A second Libyan was acquitted.
In 2003, Libya officially accepted responsibility for the attack and agreed to pay relatives of each bombing victim at least US$5 million.
Associated Press Writer Raphael Satter contributed to this report.