Brazilians will vote on Sunday in a polarised presidential race that could result in the election of a far-right former Army captain, whose praise of past dictatorships enrages critics but whose promise of a brutal crackdown on crime and corruption has electrified his supporters.
Bolsonaro, a 63-year-old former paratrooper vowing to crush crime in Latin America's biggest nation, received 46 percent of ballots - below the 50-percent-plus-one-vote threshold required for a first-round win, according to an official count of virtually all votes.
But little in this election has gone to plan, and Mr Bolsonaro's strong showing reflects a yearning for the past as much as a sign of the future. Some recent polls have projected he could beat Bolsonaro in the second round. In a broadside against Bolsonaro, who frequently talks about liberalizing gun laws, Haddad said: "We don't carry guns".
An exit poll also showed one of Bolsonaro's closest aides, former police Major Olimpio Gomes, scoring a surprise win in the Senate race in Sao Paulo state.
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.
"I think Haddad needs a bit of a miracle, it could be very, very hard for him [to win the presidency]", Oliver Stuenkel, a professor of worldwide relations at the Brazil-based Getulio Vargas Foundation higher education institute, said.
"In the past few weeks every sort of attack against Bolsonaro was made and the only result was his growth", he said.
But a Haddad voter, Jose Dias, said it would be a "catastrophe" if Bolsonaro triumphed.
In video statement streamed live over social media he told supporters: "This was a great victory, considering we had no television time, a party that is still very small with no campaign money and I was in hospital for 30 days.
Haddad and Bolsonaro will both lead populist governments".
"But attacks on Bolsonaro have tended to strengthen him", she added.
"A Haddad voter, Jose Dias, said it would be a "catastrophe" if Bolsonaro won the right to succeed unpopular outgoing centre-right President Michel Temer".
Haddad called on Brazilians to unite behind him, warning that the 1988 Constitution that underpinned Brazil's young democracy was under threat. "They don't know what it was like under the dictatorship", he said.
Many voters also like his promises to tackle corruption and to cut climbing public debt through privatizations, as well as the devout Catholic's family-first stance.
But poorer Brazilians, who benefited most from the heyday under the Workers Party's iconic former president Luiz Inacio Lula da Silva between 2003 and 2010, want a return to good times and hope Haddad can deliver.
The former Army captain, dubbed "Tropical Trump" because of his nationalist agenda and anti-establishment tirades, won almost half the votes thanks to a surge in support sparked by growing anger at corruption and antipathy towards scandal-plagued traditional parties in Latin America's largest nation.