Heathrow airport's new Terminal Five (T5) opens to passengers today.
Green activists opposed to expanding the world's busiest international airport fear T5 is the start of an 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 the Queen.
Hundreds of demonstrators are expected to stage peaceful "flash mob" protests later this morning, activists said. They said they would not disrupt flights.
Authorities will be on heightened alert after a series of embarrassing security breaches at Heathrow.
Earlier this month, a man was arrested by armed police after running onto one of the airport's runways. In February, protesters broke into the airport and wrapped a banner around a plane.