The election comes at a critical moment for the region. Brazil is one of the last major Latin American countries still led by a leftwing government, following a wave of rightwing and far-right victories across the continent and the recent US-backed takeover of Venezuela after the capture of Nicolás Maduro.
Lula, 80, is seeking a fourth non-consecutive term. He has positioned himself as a regional counterweight to US President Donald Trump, accusing Trump of trying to interfere in Brazil's election in Bolsonaro's favour.
Flávio Bolsonaro, 45, is an ally of Trump and the son of former President Jair Bolsonaro, who is serving a 27-year sentence for attempting a coup after his failed 2022 re-election bid. Flávio Bolsonaro has adopted his father's far-right agenda and campaigned on a platform of conservative values and economic liberalisation.
The first round of voting is expected to be closely contested, with polls suggesting neither candidate may secure the outright majority needed to avoid a runoff. A second round, if required, would be held later this month.
The election is being closely watched internationally, as a victory for Bolsonaro would align Brazil with the Trump administration and accelerate the rightward shift in Latin America, while a Lula win would keep a leftwing government in power in the region's largest economy.