The actors, taking part in the "Fire Drill Fridays" protest, marched up Capitol Hill steps and failed to disperse as police had ordered, leading to their arrests.
"We are called to find something in our lives worth fighting for ... something that can lift up this nation and all its people to that place, where the heart is without fear and the head is held high, where knowledge is free," Sheen told supporters.
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Sheen is best known for his work on the long-running, critically acclaimed NBC drama "The West Wing," picking up six Emmy nominations playing President Josiah Bartlet. The group called Sheen "OUR President" in posting part of his speech online. Sheen also co-stars alongside Fonda on the Netflix comedy "Grace and Frankie."
Phoenix, who on Sunday won the Golden Globe for best actor in a movie for "Joker," is considered an Oscar favorite for the best actor award.
He told protesters that any effort to benefit the environment is helpful. Phoenix is also supporting Veganuary, a campaign to get people to move toward adopting a plant-based diet.
"I struggle so much with what I can do at times. There are things that I can't avoid. I flew a plane out here ... last night," he said. "But one thing that I can do is change my eating habits, and so I just want to urge all of you to join me in that."
Fonda has been leading the Capitol Hill protests since October, and Friday was the final one in Washington. The operation is set to move to Los Angeles.
Alexandra Bacallao reported from Washington, and David K. Li from New York
David K. Li
David K. Li is a breaking news reporter for NBC News.