Bolsonaro, 45, showcased support from Argentine President Javier Milei, an AI-generated message from his jailed father and a video address from Israeli Prime Minister Benjamin Netanyahu on Saturday.
Flavio Bolsonaro pledged tax cuts and a tougher stance on crime, and stressed his commitment to carrying forward his father's political legacy.
"Here is Bolsonaro's blood," he said in an emotional tone to supporters who waved green and yellow flags. The arena in Sao Paulo where he spoke was nearly full.
Former President Jair Bolsonaro is under house arrest for his role in an attempt to overturn the election where Lula defeated him.
Milei delivered a fiery speech criticising Lula and declaring that Bolsonaro can "save" Brazil from socialism. The Argentine president flew to Brazil the night before the event and met on Saturday with Sao Paulo Governor Tarcisio de Freitas.
"We will continue offering our testimony everywhere against the lies of socialism," Milei said.
Brazil's presidency did not immediately respond to a request seeking comment.
Former first lady Michelle Bolsonaro, Flavio's stepmother, also appeared via video message to offer her endorsement, in a speech addressed to women voters. The two recently reconciled following a public dispute that played out in videos exchanged on social media.
"Flavio, may God bless and protect your candidacy," she said in the video message.
In December, the senator secured his father's endorsement to run in this year's election. But his campaign lost its early momentum in May, after leaked messages linked him to jailed banker Daniel Vorcaro, whom he later acknowledged had financed a film about his father's political career.
Bolsonaro's popularity was also dented by the dispute with Michelle Bolsonaro and by his close ties to US President Donald Trump, who has imposed tariffs on Brazilian goods.
A Datafolha poll released on Friday showed Lula leading Bolsonaro 48 per cent to 43 per cent in a runoff, up from 47 per cent to 43 per cent in June.
Flavio Bolsonaro has struggled to forge political alliances. Centre-right parties have so far decided to stay neutral rather than endorse his candidacy.
with dpa
