Casten posted on X, formerly known as Twitter, that the protesters "blocked all modes of ingress and egress," and that he and other members inside were evacuated by armed Capitol Police.
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"You have the Constitutional right to peaceably assemble and protest. But blocking all entries to a building with multiple members of Congress in it, protected by Capitol Police officers who have lived through January 6 is putting you and other innocent people at risk," Casten wrote. "We were rescued by armed officers who did not know the protestors' intent; they knew only that Members of Congress were inside, could not leave and that protestors would not let police through. Forcing police to guess intent is irresponsible and dangerous."
U.S. Capitol Police said about 150 people were "illegally and violently protesting" near the DNC headquarters building in the Capitol Hill neighborhood of Washington.
Video posted on social media showed protesters shoving police officers and trying to grab hold of metal barricades as the officers moved in to make arrests. Many of the protesters were wearing black shirts that read "Cease Fire Now."
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Protesters included members of If Not Now and Jewish Voice for Peace, who have organized other demonstrations in Washington.
"I am grateful for the USCP's professionalism and that all were safely evacuated. I'm also keenly aware that it could have been much worse. Americans have a right to assemble and protest. But PLEASE do so at a safe distance and respect local police orders," Casten tweeted.
California Democrat Rep. Brad Sherman also posted on X, saying the protesters "grew violent, pepper spraying police officers and attempting to break into the building."
The DC Metropolitan Police Department is also assisting Capitol Police.
The Associated Press contributed to this report.