Brazilian business leaders, intellectuals and artists gathered on Thursday to read manifestos defending democratic institutions after attacks by President Jair Bolsonaro that have raised fears he could reject the results of October's presidential election.

The far-right leader and former army captain has questioned Brazil's electronic voting system and attacked Supreme Court justices who oversee elections, accusing them without evidence of favoring his leftist rival, who leads in opinion polls.

Prominent Brazilians, from top bankers to union leaders, signed a manifesto organized by Sao Paulo's confederation of industries (FIESP) expressing support for the judiciary as it prepares for the Oct. 2 election under fire from Bolsonaro.

They called the top court "the ultimate guardian of the Constitution" and said the Superior Electoral Court (TSE) has conducted the country's elections with total integrity.

Another manifesto, also read out to a cheering crowd at University of Sao Paulo's law school and signed by 800,000 people, warned that the country's democracy was under threat.

"Out with Bolsonaro," the audience shouted after the manifesto called "Letter to the Brazilians" was read in the same place as a similar manifesto was read in 1977 to denounce Brazil's military dictatorship at the time.

"In today's Brazil there is no more room for authoritarian setbacks. Dictatorship and torture belong to the past. The solution to the immense challenges facing Brazilian society necessarily involves respect for the results of the elections," said part of the manifesto read out by a law professor.

The manifesto was signed by former President Luiz Inacio Lula da Silva, who is the frontrunner in the presidential race although his advantage has narrowed in recent polls.

Bolsonaro, an admirer of former U.S. President Donald Trump who backed his false claims of fraud in the 2020 election, ridiculed the manifesto, saying he had no need to sign a letter to prove his democratic credentials.

His opponents says he is preparing the ground to contest the election results if he loses, as Trump has done since 2020.

Pro-democracy demonstrations are planned by civil society organizations on Thursday afternoon in Brazil's main cities.

"The purpose of the demonstrations is the defense of democracy and free elections and respect for the popular vote and against the threat of a coup," one of their organizers, Raimundo Bonfim, told Reuters.

Brazil's most polarized election in decades is expected to go to a second-round runoff between Bolsonaro and Lula on Oct. 30 if neither candidate win 50% of valid votes on Oct. 2.