Climate activists planned to fly toy drones around London's Heathrow Airport on Friday in a controversial effort to shut down Europe's busiest travel hub over the weekend.

The campaign by Heathrow Pause -- an offshoot of the Extinction Rebellion group backed by Swedish teenager Greta Thunberg -- had been set to kick off shortly before the international airport opened at 04:30 am (0330 GMT).

The first flights from Asia arrived at Heathrow on schedule.

"Like-minded citizens just know that what we are doing is not a criminal act," group member Sylvia Dell told AFP as she prepared to fly her own drone in the coming days.

The retired mother-of-four said she was stirred to action by a United Nations report warning governments in October they had 12 years left to stop the earth from warming to dangerous levels.

"We're the fire alarm ringing in the middle of the night telling people to wake up, your house is on fire," Dell said.

The campaigners' official aim is to prevent construction of a third Heathrow runway and to cut harmful gas emissions to a net level of zero by 2025.

- 'Painful disruption' -

Heathrow rules stipulate that it must temporarily close should a drone be spotted within its five-kilometre (three-mile) exclusion zone.

It was expected to make exceptions should this only involve miniature drones flying close to the ground and away from runways or flight paths -- the rules Heathrow Pause intends to keep.

Several still-unexplained drone sightings around Heathrow and London's Gatwick Airport shut those down for brief stretches over the winter holidays.

The group's members have met airport representatives and the London police to arrange precautionary measures aimed at avoiding accidents.

They intend to fly the miniature drones at head height at hourly intervals that aim to keep the airport closed continuously for up to five days.

"You need to make disruption painful before the authorities take notice," Dell said.

An airport spokeswoman said Heathrow agreed "with the need to act on climate change" but condemned the campaigners' methods.

"We are disappointed that they plan to continue flying drones," the spokeswoman told AFP. "It is counter-productive and criminal."

London police arrested three men and two women on Thursday to prevent "criminal activity that could significantly impact on a major piece of national infrastructure".

Heathrow Pause said two of them were handcuffed after demonstrating drone flights outside the exclusion zone to a reporting crew.

Dell said she faced "years" in jail because of two prior arrests during an Extinction Rebellion protest that ground parts of London to a halt for more than a week in April.

"Prison is a terrifying prospect," she said.

But "I find the prospect of not acting and allowing the world to collapse -- that's more terrifying for me."

Heathrow flies more than 200,000 passengers daily on around 1,300 flights.