THE UK was also on alert and some European airlines canceled flights to the United States and recalled planes already in the air.

The UK was also put on full alert.

The Canary Wharf centre, the tallest building in Britain, and Stock Exchange in London were evacuated.

And all flights into and out of the UK were set to be grounded.

Security at Gatwick Airport was stepped up, Sussex Police said.

"There is heightened security at Gatwick until the events in America become clearer," said a police spokeswoman.

"There is a meeting of senior officers at the moment."

ITN reports Canada is shutting down all airports.

World travel has been thrown into chaos. Many US-bound flights had already left the UK when American officials decided to close all US airports.

Officials at other America-serving UK airports, including Heathrow and Gatwick, are trying to cope with the travel backlog.

These airports, too, would have had passengers already in the air and heading for America, with hundreds of others due to travel later today.

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