The Brazilian real has surged on the results.
The Social Liberal Party (PSL) candidate Bolsonaro was the victor of the first election round on Sunday (October 7), failing short of the 50 percent outright majority, but nonetheless accruing 46 percent of the vote.
Bolsonaro's party took a whopping 52 seats in the lower house of Congress - up from just one in the last election - giving it 10 per cent of that house and making it the second-largest party after the Workers' Party, with 56.
"I am certain that if this hadn't happened, we would have known the name of the president of the republic tonight", Bolsonaro said in a live Facebook video feed.
With the field reduced to two candidates, some analysts see Haddad as the natural inheritor of numerous centrist votes that will be up for grabs, but the scale of Bolsonaro's first-round success means that Haddad will have little room for manoeuvre.
Better-off Brazilians have rallied to Bolsonaro's pledge to crush crime in a country where there are more than 62,000 murders each year, almost as many rapes, and frequent muggings and robberies.
Supporters of presidential frontrunner Jair Bolsonaro of the Social Liberal Party, celebrate as they wait for the first results of the vote counting.
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A combination of file photos shows presidential candidates Jair Bolsonaro (L) attending a television debate at the Rede TV studio in Osasco, Brazil August 17, 2018 and Fernando Haddad attending a televised debate in Sao Paulo, Brazil September 26, 2018. It's not clear what Bolsonaro would do to further isolate Venezuelan President Nicolas Maduro, but he once suggested creating camps for the Venezuelans who have fled the country's economic collapse. The two men will face an Oct 28 runoff.
"I think Bolsonaro will carry on doing what he's doing". "If the PT is voted into power and there is a military intervention, I would support it".
Haddad called on Brazilians to unite behind him, warning that the 1988 Constitution that underpinned Brazil's young democracy was under threat. "We want to unite all the democrats in Brazil".
His grassroots social media campaign electrified the PSL, pushed it further to the right and raised the profile of candidates such as Olimpio Gomes, a former military police major, the most-voted candidate for the Senate in São Paulo Sunday.
That and upset victories in the Rio de Janeiro and Minas Gerais governor races underscored a wave of enthusiasm for Bolsonaro's allies and a backlash against the political establishment.
Lula was blocked from the race after being sentenced to 12 years in prison on corruption charges in one of the world's biggest-ever political graft scandals.
Sunday's general election - in which new federal and state legislatures were also chosen - exposed the deep divisions generated by both candidates. "Bolsonaro will have to set aside his hate speeches and straighten out contradictions between his aides", Santoro said.