Heathrow T5 opens with heightened security

LONDON (Reuters) - Heathrow Airport's giant new terminal five opens to passengers on Thursday, with security forces on high alert after threats of eco-protests.

Green militants opposed to expanding the world's busiest international air gateway fear T5 is just the start of a vast new expansion that will see a third runway, sixth terminal and up to twice as many passengers.

The new terminal at the airport, which already squeezes through 67.3 million passengers a year, was formally opened earlier this month by Queen Elizabeth.

Hundreds of demonstrators are expected to stage peaceful "flash mob" protests at 11 a.m., activists said. They said they would not disrupt flights.

"The Flash Mob is a fun event but with a very serious purpose," said their spokeswoman Andrea Needham.

Despite the peaceful assurances, authorities will be on heightened alert after a series of embarrassing security breaches at Heathrow.