But two opinions polls released on Saturday night indicated for the first time in the campaign that Lula could fall short of the more than 50 per cent of the vote needed to hand him a second four-year term.
The ballot takes place just days after the country's worst air disaster in which 155 people were believed to have died when a passenger airliner crashed in the Amazon jungle on Friday.
More than 125 million Brazilians are voting in what is is Brazil's sixth presidential election since 21 years of military rule ended in 1985.
The Lula, 60, who rose from shining shoes in city streets to the leadership of Latin America's largest country, is facing off against Geraldo Alckmin of the centrist Brazilian Social Democracy party (PSDB) and six other candidates.
Until two weeks ago, former factory worker Lula had been cruising to a first-round victory boosted by rising wages, a sound economy, and social welfare programmes that have benefited millions of poor in this country of 185 million people.
As well as the presidential contests, Brazilians are voting for state governors, Congress and state assemblies.
Lula could lose an already fragile majority in the lower house of Congress and face a larger opposition majority in the Senate.