"We believe that the terrorists' aim was to smuggle explosives onto airplanes in hand luggage and to detonate these in flight," Scotland Yard Deputy Commissioner Paul Stephenson said.

Overnight, police arrested 21 people throughout the London area and Birmingham on suspicion of plotting a terrorist act. Police searches were continuing into the day Thursday at several locations in Great Britain.

Britain's terrorist threat scale was raised to the highest level ­– critical – designating the threat of an imminent attack. The U.S. government raised its terrorist alert level on all flights from Great Britain to red, or severe — the first time the highest level had been declared since the establishment of the terrorist alert system after the Sept. 11, 2001 terrorist attacks. The level for other flights was raised to orange, or high. Air travel restrictions include a ban on all carry-on liquids.

London's Heathrow Airport, one of the busiest in the world, was closed to almost all incoming flights, creating huge crowds and confusion at the terminal. Passengers flying out of Heathrow were banned from carrying any hand luggage or liquids, suggesting that authorities are searching for liquid explosives.

A senior U.S. counterterrorism official told the Associated Press authorities believe dozens of people — possibly as many as 50 — were involved in the plot. A U.S. intelligence official told the AP the plotters had hoped to target flights to major airports in New York, Washington and California, all major summer tourist destinations.

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 counterterrorism official told the AP, calling the plot "the real deal." United, American and Continental airlines jets were targeted, the official said.

"This plot appears to have been well-planned and well-financed, with a significant number of operatives," U.S. Homeland Security Secretary Michael Chertoff said Thursday. "It was sophisticated, it had a number of members and it was international in scope."

Chertoff said the plotters intended to smuggle liquid explosives onto planes in beverages, electronic devices and other carry-on items. As a result, the Transportation Security Administration will not allow passengers to carry liquids onto planes indefinitely, although liquids such as cosmetics will be allowed in checked baggage, he said.

"They in the final stages of planning before execution," Chertoff said of the alleged plotters. He declined to say whether any bombs had been constructed, but said the group "had accumulated the capabilities necessary" to carry out the attack.

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 in turn was hidden in checked baggage.

Lengthy probe

Peter Clarke, head of Scotland Yard's anti-terrorist branch, said the investigation that culminated in Thursday's arrests has been ongoing for several months.

"The alleged plot has global dimensions," Clarke said. "The number, destination and timing of the flights that might be attacked continues to be under investigation."

"We have been looking at the meetings, movements, travel, spending and aspirations of a large group of people," said Clarke. The investigation involved British intelligence and police forces as well as unnamed international agencies, he said.

"We think we have the main players in this particular conspiracy," British Home Secretary John Reid said Thursday. However, he emphasized that the investigation was continuing and said more arrests were possible.

A senior U.S. counterterrorism official said authorities believe dozens of people — possibly as many as 50 — were involved in the plot. The official spoke on condition of anonymity because of the sensitivity of the situation.

The British Broadcasting Corp. said police were evacuating homes in High Wycombe, a town 30 miles northwest of London, near one of the houses being searched. Police refused to confirm the report or to discuss any details of the searches.

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.

British 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.

"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 Chertoff.

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

Passengers at Heathrow and London's four other airports were met by frustration, lines and extremely high security measures. At Heathrow, nearly all carry-on luggage was banned, including eyeglass cases and electronic key fobs.

Most European carriers canceled flights to Heathrow because of the massive delays created by the new regulations on hand baggage. 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.

Passengers patient

Jay Hudson, 29, an interior designer from Los Angeles, waited at Heathrow for her husband, Andre. His United Airlines flight from Los Angeles had arrived, but had been waiting on the tarmac for more than two hours. "All I want to do is — I just want to see him," she said.

Passengers on the plane were not aware of the arrests until she told her husband via cellphone, she said.

By mid-afternoon Thursday, flight status boards at Heathrow's Terminal 3, generally used for flights to and from the United States, showed about half of all flights had been canceled or delayed.

Thousands of people crammed into the terminal were standing, sitting or sleeping on almost every available space, including tables.

Huge lines formed at ticket counters and behind security barriers at Heathrow and other British airports. At airline check-in desks, attendants gave passengers clear, resealable bags to carry the few things allowed as carry-on items: wallets, passports, toiletries and keys. Everything else had to be placed into check-in luggage.

Faced with a three-hour flight delay, Margo McIntyre, 62, of Burns Flat, Okla., described three ways to pass the time waiting for her flight: "Eat, drink, shop. We still have a few British pounds to spend."

McIntyre was traveling home after a one-week vacation in London with two relatives and a family friend. "If you're going to travel, today is the day to travel because security is so high," said Kirsten McIntyre, an Oklahoma City TV reporter and Margo McIntyre's daughter. "Today is a good day to fly."

Most people at Heathrow seemed to take the delays in stride. One couple traveling to Newark International Airport in New Jersey wasn't deterred at all.

"The only thing we were worried about was how we were going to be passing the time on a plane for eight hours with nothing to read," said 24-year-old Jill Bernstein, noting that books weren't allowed through security, although passengers could buy them from shops past the checkpoint.

Bernstein, a student who had just completed her studies in Great Britain, was returning to her hometown of Hoboken, N.J. with boyfriend, Trevor Knott, a 31-year-old accountant from Brighton, England.

"We're actually moving there today," Bernstein said.

"Or maybe tomorrow," Knott half-jokingly added.