At least five animal rights activists rushed the stage as presidential candidate Bernie Sanders spoke at a rally in Oakland, California, video posted online shows.
The protesters were led away by several Secret Service agents before they were able to reach the Democrat, whose speech was briefly interrupted by the disturbance, a video posted by CNN showed.
A Secret Service agent rushed to Sanders' side, grabbed him and wrapped his arms around the presidential candidate during the ruckus.
"We don't get intimidated easily," said Sanders, as he resumed his speech.
Animal rights group Direct Action Everywhere claimed responsibility for the protest, saying in a statement that it hoped to persuade Sanders to take a tough stance against factory farms.
"Bernie Sanders claims to support the 'good' farms," activist Rachel Ziegler said in the statement. "But as our repeated investigations have shown, even the 'good' farms are horrific."
The organisation posted a video online showing at least five protesters being led away by security personnel after they climbed on to the stage on Monday.
Sanders, a US senator from Vermont, is running far behind Hillary Clinton in the race for the Democratic nomination for the November 8 presidential election.
In March, a man stormed the stage as the presumptive Republican presidential nominee Donald Trump spoke in Vandalia, Ohio before being held by several agents.