The approximately 40 people swept past security guards and broke a glass door to get in the lower house of Congress chamber, interrupting the start of the session.
Taking over the podium they chanted “general here, general here,” in reference to support for a takeover by the army.
Brazil was ruled by a military government from 1964 to 1985 and a small minority of Brazilians support the idea of a new takeover in response to widespread corruption and a floundering economy.
The chamber was mostly empty at the time, but the scenes of shouting protesters were played on national television. All the demonstrators were detained by police for questioning.
“It’s worrying and serves as a warning. We are returning to an era of extremes,” said one congressional deputy, Betinho Gomes.