Mark Ruffalo, Shailene Woodley We're Here To Fight the Good Fight ... Bear Witness to Our Peaceful Protest!

10/24/2016 7:06 AM PDT

Mark Ruffalo is 100% behind Shailene Woodley's protest of the Dakota Pipeline, and thinks she needs to get back to the front lines of the battle ... sans another arrested. 

We got Mark at MacArthur Park Sunday where he, Shailene, Susan Sarandon and hundreds of others gathered to protest the oil pipeline. Mark tells our photog Shailene's arrest was BS ... and says it hints at a bigger problem in the country.

Shailene gives her two cents as well, making it clear ... she's in this for the long haul.