Bailiffs are trying to remove an environment protester from an underground tunnel at the site of a housing scheme.
The protesters built platforms in the trees
Three people were arrested as bailiffs moved onto the site in Debdale Lane, Mansfield Woodhouse, Nottinghamshire.
The group had been camped in the threatened trees for 15 months.
Around 50 bailiffs, along with 70 police officers who were there to ensure safety, moved in at first light on Monday morning.
Three protesters were arrested by the bailiffs for on suspicion of obstructing a court officer.
A fourth is understood to have left voluntarily.
The bailiffs say safety is their main priority while the remaining protester is located.
A report into the health of the trees has indicated they need to be felled and a judge has ruled they can be removed.
Developer Bellway Homes says the new project will include six hectares of open space with hundreds of trees.