The 22-year-old man had locked himself to the underside of a car. After he was freed he was arrested.
The Register Guard reports two other demonstrators also were arrested and all three were jailed on disorderly conduct charges.
They were part of a group holding signs and chanting "Save our forests, save our lungs" outside the Seneca Sustainable Energy plant.
The demonstration took place as Seneca was conducting a tour of the $50 million facility that opened in February. It sells power to the Eugene Water and Electric Board.