Climate campaigners' plans to use drones to disrupt flights are "criminal and counterproductive", Heathrow Airport said.
Heathrow Pause, an Extinction Rebellion offshoot, are to fly toy drones within the no-fly zone, grounding all flights, on 13 September.
Organisers expect up to "a couple of hundred people" to be involved.
The Met Police said it would "do everything in its power to stop and prevent any such criminal activity".
The activists said they "can't stand aside" and have to act, with disruption intended to last for a number of days and potentially longer.
A Heathrow spokeswoman said airport chiefs "agree with the need to act on climate change", adding: "This is a global issue that requires constructive engagement and action.
"Committing criminal offences and disrupting passengers is counter-productive."
Heathrow Pause said the small, lightweight drones would not be flown above head height or on flight paths.
Flying drones over 15.5lbs (7kg) in weight above 400ft (122m) is considered "hazardous" under current legislation.
However, activists plan to operate within the three-mile (5km) no-fly zone surrounding the airport, introduced after drone sightings at Gatwick Airport in December caused about 1,000 flights to be cancelled or diverted.
The group said the airport would be given one hour's notice before each drone flight. The flights are planned at regular intervals, ensuring "no aircraft flights will take place".
Extinction Rebellion said it did "not support" the group's actions, although both groups share many of the same coordinators.
In a statement the group said: "Extinction Rebellion UK neither condones nor condemns the action and will not be supporting it in any way."
The Met's Deputy Assistant Commissioner Laurence Taylor said: "Heathrow Airport is part of our national infrastructure and we will not allow the illegal activity of protesters to cause misery to thousands of air travellers.
"The dangers of flying drones in airspace used by airliners carrying passengers are stark and the consequences potentially very severe.
"We would urge anybody intending to join this event with a view to committing criminal activity, whether considered peaceful or not, to think twice."
In a 22-page manifesto, Heathrow Pause said protesters "are prepared to be arrested peacefully".
"Our readiness and courage are based on the conviction that our actions are a humanitarian act, this is an act of conscience, and this action is proportionate," the group said.