It is the final electoral battle for Lula, who turns 81 on Tuesday and is hoping to pull off a dramatic turnaround after coming in second to the son of former president Jair Bolsonaro in the first round of voting.
Final polls showed the two candidates neck-and-neck.
Sunday’s runoff will determine whether Brazil remains one of Latin America’s few leftist nations or joins a regional swing to the right championed by US President Donald Trump, an ally of the Bolsonaro family, though he has not openly endorsed Flavio.
Lula, who is seeking a fourth non-consecutive term, has pitched himself as a bulwark against US interference and democratic backsliding and promised to expand social protections.
The right “wants to govern with the American flag,” he said after casting his ballot in the state of Sao Paulo.
Flavio Bolsonaro, 45, has vowed to crack down on crime and slash government spending, while handing Brazil to “the Lord Jesus Christ.”
“The time has come for real change,” he declared after voting in Rio de Janeiro. “Brazil can’t stand another four years like this.”
Bolsonaro presents himself as more moderate than his firebrand father, who is serving a 27-year sentence for attempting a coup after his 2022 election loss.
Lula, revered for social welfare programs that lifted millions out of poverty, made a dramatic comeback to office in 2022 elections after a corruption scandal saw him jailed for over 18 months.
Polling stations in Brazil will close at 4.00am Hong Kong time, with results expected within hours.