Sunday's demonstrations, organised by social groups, artists, and political parties, denounced politicians for trying to soften the punishment for the former president, who was convicted in September to 27 years in jail, and for his supporters who stormed government buildings following his defeat.
It was the first major protest since Bolsonaro, whose far-right movement reshaped Brazilian politics, started serving his prison sentence last month in a cell that was made for him at a federal police building in Brasilia, the capital.
According to researchers from the University of Sao Paulo, about 15,000 people gathered in Sao Paulo, Brazil's biggest city.
Last week, a conservative majority in the lower house of Congress voted to pass a bill that could cut Bolsonaro's sentence to just over two years, according to its sponsor. The bill still needs to be considered by the Senate.
Claudio Pfeil, a teacher who was among an estimated 20,000 protesters on Copacabana Beach in Rio de Janeiro, said he was demonstrating to show that Brazilians will not accept leniency for those who tried to undermine democracy.
"All of them are paying for their crimes," he said. "We won't accept even a centimetre of retreat."
The Supreme Court's conviction of Bolsonaro and his co-conspirators in the government and armed forces marked the first time Brazil has punished military officers for attempting to overthrow democracy in a history stained by violent coups.
The current bill also would soften the punishment of Bolsonaro's supporters for taking part in the 2023 storming of government buildings in Brasilia. But Bolsonaro, who is now 70, would still be barred from office for another three decades.
Leftist Brazilian President Luiz Inacio Lula da Silva, who defeated Bolsonaro in the 2022 elections and is planning to run for re-election next year, is expected to veto the bill if the Senate passes it. But Congress could override a veto, an outcome likely to trigger challenges in the Supreme Court.
