At least four people on Monday rushed the stage as presidential candidate Bernie Sanders spoke at a rally in Oakland, where Secret Service agents swarmed around him and detain the protesters, CNN reported.
The people were quickly led away by several 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 intimated easily," said Sanders, who seemed unfazed by the situation, as he resumed his speech.
It is unclear how many people were detained and why they were protesting. At least four were seen on the CNN video being led away by agents, who lifted one man up by his feet and carried him away.
Sanders, a US senator from Vermont, is running far behind Hillary Clinton in the race for the Democratic nomination for the Nov. 8 presidential election.
In March, a man stormed the stage as the presumptive Republican presidential nominee Donald Trump spoke in Vandalia, Ohio before he was met by several agents.