Tents, stages and speakers were set up in Windsor Home Park on Friday morning ahead of an “action-packed” weekend of activities.
One protester said she thought the King would “probably be here” and “up for doing some camping” if he was not the head of state.
No direct action or disruption to the public is planned – but Windsor council has said they do not support nor permit the three-day campsite in the council-run parkland.
The group has been “liaising” with local authorities, said spokesman Etienne Stott, 45, who won a gold medal at the London 2012 Olympics in the canoe slalom.
On Friday afternoon, two activists wearing masks of Charles and Queen Camilla paraded around the makeshift camp for photographers.
Activist Marijn van de Geer, 41, said of Charles: “I reckon if he wasn’t the King he’d probably be here.
“I think he’d probably be quite good at giving some talks and trainings.
“He’s quite an outdoorsy person, he’d probably be up for doing some camping, it’s a shame he’s the King because he obviously can’t be here.
“As an individual he has spoken out a lot about the environment.
“He was well ahead of his days at the time when he was talking about it and everyone was laughing at him.
“This is 100% not an attack at the King.
“Extinction Rebellion UK does not have any plans to storm Windsor Castle, any hotels, nor cause public disruption.”
On Saturday, activists are planning to march to Windsor Castle, arriving at noon.
At 2:30pm, the “Massembly” will take place on the Long Walk, finishing at 5:30pm. The “fUnfair” will re-open at 6pm.
@sloughobserver Not the real Charles and Camilla, before anyone accuses us of misinformation. We’re here at the Extinction Rebellion event near Windsor Castle. You can read more here: https://t.co/Z7LgnDE0Po pic.twitter.com/1AzfxA66qR
— Richard Lemmer (@RichardLemmer89) August 30, 2024
The centrepiece of the weekend takes place on Saturday afternoon – a mass assembly or Massembly which “should be one of the largest participatory, democratic events ever held in the UK.”
Thames Valley Police has said the force “will not tolerate criminal behaviour and those choosing to commit offences can expect to be dealt with swiftly and robustly.”
Discussing concerns about direct action, Mr Stott wanted to “reassure” people that the event was family friendly and “disciplined”
He said: “All different groups have their different ways of doing things. We’re part of an ecosystem of groups. Just Stop Oil have got their niche and particular way of doing things. We all need to communicate the lack of action from governments.”