Brazil's Senate has passed a bill to slash the prison term of far-right ex-president Jair Bolsonaro, who was convicted of plotting a coup after losing reelection to left-winger Luiz Inacio Lula da Silva.

Passed by the lower chamber last week, the bill now heads to Lula, whose expected veto could eventually be overridden by Congress, following its passage on Wednesday by a 48-25 margin in the Senate though political forces are more evenly balanced in the upper house.

Bolsonaro, 70, began a 27-year prison sentence in November, and under current rules was expected to serve at least eight years behind bars before becoming eligible for a looser regime.

The new legislation, which changes how sentences are calculated for certain crimes, could, however, see Bolsonaro serve a little over two years in prison.

Following months of jockeying by Bolsonaro's supporters in Congress for some sort of amnesty, the bill moved surprisingly quickly through both the conservative-controlled lower chamber and the more balanced Senate.

Bolsonaro's oldest son Senator Flavio Bolsonaro – whom the former president anointed as the candidate of the right in 2026 elections – said in a video "it wasn't what we were fighting for, but it's what was possible at this juncture."

The bill provoked protests in cities across Brazil on Sunday, where demonstrators chanted "no amnesty" and held up banners reading: "Congress, enemy of the people."

Centrist senator Renan Calheiros slammed the vote as a "farce" and walked out of the session, accusing the government's allies in parliament of letting the vote take place in exchange for support for a budget initiative as part of a backroom deal.

Senators have amended the wording of the bill to limit its benefit, after fears that it could ease punishment for a wide variety of criminals.

It is specifically aimed at benefiting those convicted as part of the coup plot, as well as more than 100 Bolsonaro supporters who were imprisoned for their role in January 2023 riots against the seats of government in Brasilia, shortly after Lula took office.

Bolsonaro is serving his sentence in a special room at a police facility in the capital Brasilia, after a dramatic start to his jail term when he took a soldering iron to his ankle monitoring bracelet while under house arrest.

Lula has vowed to veto the bill, saying Bolsonaro "must pay" for his crimes.

"This bill is destined to be vetoed" by Lula, said senator Randolfe Rodrigues of Lula's leftist Workers' Party.

However, Congress has the last word and can overturn the president's veto.

Bolsonaro was convicted for a scheme to stop Lula from taking office after his razor-thin loss in a bitter 2022 election that highlighted stark political divisions in Brazil.

The plot allegedly involved a plan to assassinate Lula, his vice president Geraldo Alckmin and Supreme Court judge Alexandre de Moraes.

Prosecutors said the scheme failed because of a lack of support from military top brass. (AFP)