Smiling with the military top brass at his side, the far-right leader stood atop the palace steps as a long convoy of tanks and armored vehicles filed through the Brazilian capital.

Critics said the display was reminiscent of Brazil's 1964-1985 military dictatorship, for which Bolsonaro, a former army captain, is openly nostalgic.

"This is pathetic... Our democracy has the means to defend itself from a coup-mongering outburst," said Senator Omar Aziz, chair of a Senate committee investigating the government's widely criticized handling of Covid-19.

Bolsonaro, 66, is also facing investigations over his attacks on Brazil's electronic voting system, which he alleges -- without evidence -- is riddled with fraud.

The Supreme Court and Supreme Electoral Court have ordered him investigated for potential crimes in his unrelenting campaign against the system, which Brazil has used since 1996.

The military parade came on the same day Congress defeated a Bolsonaro-backed bill to overhaul the system.

The president wants "printable and auditable" paper ballots for every vote cast, in addition to the electronic system.

But the bill got only 229 of the 308 lower chamber votes it would have needed to progress.

Bolsonaro is trailing in the polls for the presidential election in October next year.

Opponents accuse him of setting the stage to claim widespread fraud if he loses, following in the footsteps of his political idol, former US president Donald Trump.

Bolsonaro insisted there was no link between the military parade and the bill debated before Congress.

Officially, the event was for the armed forces to give the president an invitation to upcoming military exercises outside the capital.

However, it is the first time tanks have rolled past the presidential palace, Congress and high court since the dictatorship era.

"The president is using this tank parade to try to intimidate Congress and the Supreme Court," said political scientist Mauricio Santoro of Rio de Janeiro State University.

"He wants to show the armed forces are on his side."

Nine opposition parties issued a joint statement condemning the display, including the Workers' Party of former president Luiz Inacio Lula da Silva, who leads Bolsonaro in the polls for 2022.

Soldiers arrested a group of protesters who tried to block the convoy.

A small crowd of Bolsonaro supporters also attended, some carrying signs calling for the military to intervene to "save Brazil."