A California man who attacked police officers at the Capitol on Jan. 6, 2021, in a sustained assault with his hands, his feet, a flagpole, crutches, pepper spray and broken pieces of furniture was sentenced on Friday to 20 years in prison — one of the stiffest penalties issued in more than four years of prosecutions of the rioters.

The man, David Dempsey, was so aggressive that at one point he assaulted a fellow rioter who was trying to disarm him. Mr. Dempsey, described by prosecutors as a former construction worker and fast-food employee with a “very significant history of arrests,” also stood beside a gallows outside the building and called for the hanging of prominent Democrats, including Speaker Nancy Pelosi and former President Barack Obama.

“Dempsey was one of the most violent rioters, during one of the most violent stretches of time, at the scene of the most violent confrontations at the Capitol,” prosecutors wrote in court papers filed this month.

The only rioter to have faced more time in prison than Mr. Dempsey so far is Enrique Tarrio, the former leader of the Proud Boys, who was sentenced in September to 22 years in prison after being convicted on charges of seditious conspiracy. Mr. Dempsey’s 20-year term was higher than the sentences handed down to one of Mr. Tarrio’s lieutenants, Joseph Biggs, who was given 17 years, and to Stewart Rhodes, the leader of the Oath Keepers militia, who is serving an 18-year term.

Prosecutors say that Mr. Dempsey, who pleaded guilty to assault in January, climbed atop other rioters at the Lower West Terrace of the Capitol, using them like “human scaffolding” to get at the officers protecting the entrance to a tunnel there.