Campaign group Extinction Rebellion pledged on Thursday to “escalate” demonstrations, which have so far focussed on busy roads in the centre of the capital.
Organisers confirmed there would be protest action at Heathrow, Britain’s busiest airport, on Good Friday after an appeal for activists willing to take part was posted on WhatsApp.
Heathrow said it was “working with authorities” to prepare for any action over Easter.
But HuffPost UK understands there has been a groundswell of anger towards the proposals within Extinction Rebellion itself.
A spokesperson for the group, Jayne Forbes, said reaction had been “almost universally negative” towards the plans.
“People are saying we should do it during the week to target business travellers rather than families going away this weekend,” Forbes told HuffPost UK.
“We don’t want to alienate our supporters.”
Forbes said she had received the message, which was leaked onto social media.
“Tomorrow we raise the bar. We are going to shut down Heathrow,” the message said.
It added: “There is deep remorse for those whose holiday and family plans will be disrupted tomorrow. It is not our intention to cause further separation.
“However, the aviation industry needs to be targeted and we are all aware of the deep structural change that needs to come.”
“We are all going to face pushback on this,” the message conceded.
Millions are set to take to the road, sea, and skies for holidays over the Easter weekend, which is this year expected to be one of the busiest on record for airports.
Extinction Rebellion’s co-founder, Dr Gail Bradbrook, said on Thursday: “There’s a key piece of a rebellion where you have to escalate your tactics.”
The group has had “very heated discussions about what’s for the best” in terms of its methods, she said.
“It’s certainly an option that tactics will be escalated if our demands are not met,” she said.
Asked about whether key sites like Waterloo Bridge will remain held for two weeks as planned, she said: “It’s in the hands of the gods really isn’t it? It’s up to whether people respond to the call to come onto the streets.
“More people are joining us all the time. We’re having a fantastic time here. It’s how life is meant to be lived with people coming together and actually exerting their political power.”
Extinction Rebellion is demanding that governments take necessary action “on the global climate and ecological emergency”.
The group’s action this week has seen hundreds arrested across the capital. Three people were denied bail on Thursday after being charged with offences following disruption on the Dockland Light Railway.
Organisers said they expect “hundreds” more activists to join its campaign over the Easter weekend.
Scotland Yard said: “We have strong plans in place that will enable us to deploy a significant number of officers to Heathrow and take firm action against any protester seeking to cause disruption at this location by committing criminal offences such as obstruction of the highway.”
A Heathrow spokesperson said: “We are working with the authorities to address any threat of protests which could disrupt the airport.
“While we respect the right to peaceful protest and agree with the need to act on climate change, we don’t agree that passengers should have their well-earned Easter Break holiday plans with family and friends disrupted.
“Passengers should contact their airlines for up to date information on their journeys.”
Extinction Rebellion later issued a statement on the Heathrow plan which appeared to be a series of quotes about climate change from Sir David Attenborough.