Thunberg spoke alongside Tokata Iron Eyes, a 16-year-old activist from Pine Ridge who planned the rally and invited Thornberg to speak at the Pine Ridge and Standing Rock reservations.

Iron Eyes and Thunberg met at an Amnesty International event in Washington, D.C. last month and Iron Eyes invited her new friend to visit her homelands, according to a news release from the Lakota People's Law Project. The pair spoke together at a youth climate crisis panel Sunday in Pine Ridge and will hold another one Tuesday in Fort Yates, North Dakota on the Standing Rock Reservation.

"We are marching for our lives, we are marching for climate justice and we are marching for indigenous rights at the same time — because those two things go hand in hand. There's no one without the other," Iron Eyes said. "Indigenous people need to be in the forefront of the climate movement because we are the frontline communities who are suffering the most from this crisis and Greta knows that."

Between 300-400 people attended the rally in Rapid City, according to police spokesman Brendyn Medina. Many held handwritten signs with slogans such as "listen to the scientists," "water is life," "there is no Planet B," "climate justice is indigenous," and "you'll die of old age, we'll die from climate change."