Police estimated about 250 people gathered in front of the building in Charlotte, loudly but peacefully opposing the $2.4 billion, 800-megawatt generator. Organizers said 44 protesters were arrested for trespassing after walking across a line set up by police.
A coalition of environmental groups which is fighting the project in court and in appeals to state air quality regulators said they're trying to draw attention to their efforts.
"We absolutely accomplished everything we wanted. We demonstrated that we have broad base support here in Charlotte and North Carolina and the country to stop the proliferation of coal-fired plants," said John Deans, the Greenpeace USA field organizer working with the Cliffside Coalition.
"It's a severe threat to the environment. We have to shut them down."
Environmentalists argue the $2.4 billion coal-fired generator will pump tons of carbon dioxide and other pollutants into the sky, but Duke Energy said new technology will reduce the pollutants emitted and their environmental impact.
Company spokeswoman Marilyn Lineberger said Duke respects free speech but has zero tolerance for illegal activities.
Lineberger said the generator is about 30 percent complete at the company's Cliffside plant, about 40 miles west of Charlotte.
Protesters, some of whom were holding signs that read "Save Our Mountains" and "No New Coal," said they came out because of their deep concern for climate change.
Among those arrested was 86-year-old Betty Robinson of Charlotte, who said she wanted younger generations to live as long as she had in a healthy environment.
"They need to stop building this. It has to be stopped," she said, echoing calls from other protesters.
Avran Friedman, 59, was the first person arrested and said he was protesting to help his family.
"I'm doing it for my children," Friedman, clad in a gray suit, said as an officer placed him in handcuffs.
"This is a real threat," he added. "How can we just stand by and watch this go up and not do a thing?"
Charlotte-Mecklenburg Police Officer Robert Fey said the amount of time those arrested would have to stay in jail largely depends on whether they have a criminal record.