The co-founder of Extinction Rebellion has been convicted of causing £27,500 of damage after a four-year legal saga over protest rights.
Gail Bradbrook was unanimously found guilty of criminal damage to the Department for Transport in 2019 after a three-day trial.
At one stage, the judge warned he could use anti-jury tampering powers amid rows over what she could tell jurors.
She said she was silenced by the court - and will be sentenced next month.
The Extinction Rebellion mass demonstrations movement staged two major protests in London in 2019 - leading to thousands of arrests and a policing bill running to ten of millions after parts of the city were brought to a halt.
During the second protest in October, Gail Bradbrook climbed onto an entrance canopy at the Department for Transport's headquarters.
She then used tools to break a large pane of reinforced security glass. The specialist glass cost £27,500 to replace because it had to meet specific security standards and had to be quickly replaced.
Bradbrook said she had specifically chosen to target the DfT because of the huge environmental damage that was being caused by the
HS2 project.