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...