Several activists from the Chilean contingent of Greenpeace unfurled a giant sign Tuesday on the front of the Santiago office of Brazilian energy firm CGX to protest the "illegality" of a coal-fired power plant the company plans to install in northern Chile.

With this action, the organization demanded an end to the Central Castilla project, located some 800 kilometers (500 miles) north of Santiago in a coastal area teeming with sea turtles, seals and penguins.

The activists hung a giant banner that read "Castilla is illegal" on the CGX corporate headquarters.

The environmentalists infiltrated the 20-story building and lowered themselves from the rooftop to install the sign.

"This project only exists to wrap up a multimillion-dollar deal and to provide cheap energy to mining companies without a thought for its environmental and social costs," the executive director of Greenpeace in Chile, Matias Asun, said in a communique.

With an investment of $4.4 billion, Central Castilla plans construction on the coast at Punta Cachos in the Atacama region.

Colonies of sea turtles, penguins and seals thrive in the area, while on the sands of the interior a phenomenon called the "flowering desert" is occasionally seen, brought to life in some years by light winter rains.

Residents near the site of the proposed plant are challenging the project in court.

The Castilla coal-fired plant intends to cover between 10-15 percent of the new power generation requirements of Chile's principal electrical grid.