"We've been doing something called Fire Drill Fridays to call attention to the climate crisis," Fonda said. "We got arrested for civil disobedience."
Tomlin jokingly said she "learned a lot in the slammer," with Fonda adding that Tomlin was only jailed for two hours.
"I want to give a shoutout to my cellmate, Skittles," Tomlin said. "Hey, Skittles! Yo mamma."
Speaking seriously, Fonda told the audience that "nothing's better" than being a part of the climate change movement.
"We march, we chant, we make our voices heard," she said.
Fonda and Tomlin started a chant in the audience before being led off stage by two police officers. The pair later promoted the first Fire Drill Friday in Los Angeles, which will take place Feb. 7 at Los Angeles City Hall.
"We've brought it to California, and we're bringing it all around the country," Fonda said.
Fonda and Tomlin play the title characters, Grace Hanson and Frankie Bergstein, in Grace and Frankie. The series was renewed in September for a seventh and final season.