Protesters against the HS2 high-speed rail scheme have clashed with police in woodland that inspired Roald Dahl.
Officers and security staff are trying to remove about 40 activists from Jones' Hill Wood in Buckinghamshire, including 15 in makeshift treehouses.
Campaigners have opposed the building of the London to Birmingham route through the woods for seven months but eviction teams arrived on Thursday.
A spokesperson for HS2 called it a "straight-forward issue of trespass".
Three arrests have been made so far by Thames Valley Police.
Demonstrators range from teenagers to pensioners and their eviction is expected to take several days.
Roald Dahl wrote his stories in the Buckinghamshire town of Great Missenden, and classics like Fantastic Mr Fox are said to have taken inspiration from the wood.
Steve Masters, 50, has slept there every night for three months and said he was protesting against climate change, the loss of wildlife habitats and to protect "the literary memory of Roald Dahl".
"It (HS2) is an engineering project that's not needed, it's not going to be carbon neutral for 120 plus years," he said.
"I won't be walking out of this wood, they will have to force and carry me out. I'm even prepared to go to prison."
HS2 claimed the protests were a safety danger and put unnecessary strain on the emergency services.
"By providing a cleaner, greener way to travel, HS2 will help cut the number of cars and lorries on our roads and help the country's fight against climate change," a spokesman said.
The company added it had planted more than 70,000 new trees in Buckinghamshire to create new wildlife habitats.
Construction work officially began last month on HS2, which is set to link London, Birmingham, Manchester and Leeds.
Prime Minister Boris Johnson said it would "fire up economic growth and help to rebalance opportunity".