Portugal and Spain beefed up airport security and suspended flights to London's Heathrow airport on Thursday after the British authorities thwarted a plot to blow up planes in mid-air.

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.

Spain 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 (AENA).

Spanish airline Iberia said it had canceled all its flights to Heathrow, the only British airport it flies to, and those flights from London to Spain were experiencing long delays due to additional security checks.

The AENA urged all passengers to check their flights prior to the scheduled time with their airlines.

Earlier on Thursday, British Home Secretary John Reid said Britain had foiled an attempt to detonate explosive devices smuggled aboard as carry-on luggage on midnight flights from Britain to the United States. Twenty-one suspects have been arrested. London's Heathrow airport is now closed to all incoming flights. All British airports have implemented additional security measures and reduced carry-on luggage to a minimum. Documents, glasses and indispensable items have to be carried in transparent plastic bags.

(Xinhua News Agency August 11, 2006)