Lucy Lawless -- Belting Out 'Xena' War Cries During Oil Tanker Protest

Lucy Lawless is currently strapped to the top of an oil tanker in New Zealand ... screaming "Xena" war cries from the top of her lungs in an effort to halt drilling plans in the Arctic ... and TMZ has it all on tape.

Lawless and six other members of Greenpeace snuck on to the ship today while it was docked in Port of Taranaki ... and now they're REFUSING to leave until the captain agrees to call off the mission.

But even rebels carry cell phones ... and TMZ just spoke with Lucy who tells us she's been unleashing "Xena" battle cries during her protest ... telling us, "I just can't help myself."

Lawless explains, "We know we're going to be arrested ... but ... this is a moral decision for us."

Lucy tells us she's very passionate about protecting the "fragile" ecosystem in the Arctic ... and hopes her efforts will pay off.