Xinhua - English

BEIJING, Aug 11 (Xinhua) -- Airports in large parts of the world have stepped up security measures after British authorities thwarted Thursday a plot to blow up U.S.-bound planes in mid-air.

After Britain foiled an attempt to detonate explosive devices smuggled aboard flights from Britain to the United States, airports in Britain, the United States, Portugal, Spain, France, Canada, Russia, Poland and some other countries have all raised their security levels.

The London Heathrow Airport is now closed to all incoming flights following the threat.

In the United States, the Department of Homeland Security raised the national threat level to its highest rung -- severe, or red -- for commercial flights from Britain, and to high, or orange, for all flights within the United States.

Major U.S. cities all strengthened security at airports. At the Los Angeles International Airport, passengers are requested to throw away or place in checked luggage liquids, gels, perfume vials and even water bottles and toothpaste tubes.

At the airport of Lisbon, capital of Portugal, passengers had been waiting for flights to Britain since early Thursday with little idea on when the flights would be resumed after the airport authorities said all flights to London were canceled.

In Spain, authorities canceled some flights to London and delayed some others. Only 80 flights of the 787 scheduled flights to Britain for Thursday had operated as normal, said the Spanish Airports and Air Traffic Authority.

French Interior Minister Nicolas Sarkozy announced Thursday France will "search at 100 percent" all the carry-on luggage on the flights to the United States, Britain and Israel. Some 50 flights from France to London were canceled the same day.

A number of other countries, including Russia, Canada and Poland, also followed suit by upgrading security at airports due to the foiled attempt which U.S. officials said could cause a disaster comparable to the Sept. 11 attacks.

Earlier on Thursday, both U.S. and British officials confirmed that at least 21 suspects in London were in their final stage of plotting to blow up several U.S.-bound flights from Britain with liquid explosives they planned to take on board.

Reports said the plot targeted at least three major U.S. carriers on flights from Britain, with destinations including at least five major U.S. cities.

U.S. Homeland Security Secretary Michael Chertoff told a morning press conference that the plot was "as sophisticated as any we have seen in recent years as far as terrorism is concerned."

He said the foiled plot would have been a disaster on a scale comparable to the Sept. 11 attacks five years ago.

We could have seen "hundreds maybe thousands of people being killed" if the plot had succeeded, Chertoff said in a PBS interview. Enditem