The star-studded lineup at the Coachella music festival included an unexpected guest Saturday night.

In a surprise appearance before a massive crowd at the Indio, California, festival, Sen. Bernie Sanders took the stage to deliver a rousing speech blasting President Donald Trump and exhorting the young audience to stand up against billionaires, the fossil fuel industry, “greedy insurance companies” and states that are rolling back abortion rights across the nation. As he spoke under a full moon, hundreds of screaming fans rushed over, cameras in hand, to capture the senator’s unannounced address, which followed a blockbuster set from Charli XCX on a neighboring stage. Sanders had been invited by festival organizers to introduce singer-songwriter Clairo, whom he praised for her own record of activism. He was introduced by Maxwell Frost, the Florida Democrat who recently became the first Gen Z member of Congress.

“I’m here because Clairo has used her prominence to fight for women’s rights, to try to end the brutal war in Gaza, where thousands of women and children are being killed” Sanders said when he took the stage. He used the opportunity to launch into an emotional address warning of the dangers facing the world since the return of the Trump administration, and told the cheering crowd that “the future of what happens in America is dependent on your generation.”

The Vermont senator’s first mention of the president prompted a raucous chorus of boos from the crowd. “I agree!” Sanders responded. “He thinks that climate change is a hoax. He is dangerously wrong and you and I are going to have to stand up to the fossil fuel industry and tell them to stop destroying the planet.”

He emphasized that since assuming office in January, the administration has taken multiple steps to dismantle climate efforts, including mass staff cuts at environmental agencies and the rollback of environmental regulations.

“Now you can turn away and you can ignore what goes on, but if you do that, you do it at your own peril,” the 83-year-old senator and two-time presidential candidate continued as the crowd continued to cheer. “We need you to stand up to fight for justice. To fight for economic justice, social justice and racial justice.”

He urged the audience to resist oligarchs like Elon Musk who had hijacked the American political system. “We have an economy that is working very well for the billionaire class, but not for working families,” Sanders said, concluding his three-minute address with an attack on the drug companies and insurance companies who do not understand “that health care is a human right.”

Sanders’ appearance at the festival followed a Los Angeles rally he had held earlier in the day as part of his “Fighting Oligarchy” tour with Rep. Alexandria Ocasio-Cortez of New York. Organizers said 36,000 people had attended the duo’s L.A. appearance, which featured performances by Neil Young, Joan Baez and Maggie Rogers. It was the latest stop in a national anti-Trump tour that has drawn massive crowds across the country.