Bolsonaro, a former Army captain and veteran lawmaker, almost won the presidency outright on Sunday, taking 46 percent of votes against leftist Fernando Haddad's 29 percent, part of a swing to the right in Latin America's largest nation. If Bolsonaro - who was stabbed during the election - manages more than 50 percent, he will win the presidency outright.
Sunday's exit polls called Jair Bolsonaro on 45 percent of votes and Fernando Haddad on 28 percent, which was proved true, considering the margin of error of 2 percent. In the weeks ahead of an October 28 runoff against former Sao Paulo mayor Fernando Haddad, Bolsonaro's main proposals are sure to come under much scrutiny. The Congressman will head toward a second-round runoff against Haddad, the former Mayor of São Paulo, who so far has gained 27.9 percent of the popular support. The world's fifth most populous country has been roiled by years of rising crime, recession and graft scandals.
"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.
Still, preliminary results showed unexpectedly big congressional wins by Bolsonaro proxies including former military police Major Olimpio Gomes, his campaign manager in Sao Paulo, who was elected to Senate.
Critically, he's upended the traditional political campaign approach, staying clear of public debate over big policy issues and instead mobilising social media to appeal directly to voters with plainly worded pledges.
Haddad may yet gain the backing of other candidates in the race, but that might just feed Bolsonaro's criticism that traditional politicians are only interested in protecting their own.
Senna said he was anxious that Bolsonaro's presidential rivals would gang up on him and back Haddad in the runoff.
"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 global relations at the Brazil-based Getulio Vargas Foundation higher education institute, said.
Reflecting confidence that he will win the second round, Bolsonaro said he had already begun talks with other lawmakers in Congress to build an eventual governing coalition.
Some voters - particularly women - wore "Not Him" slogans to polling stations, declaring their fierce opposition to Bolsonaro.
"This is a victory for honest people who want the best for Brazil", said Bianca Santos, a 40-year-old psychologist who gathered outside a hotel where Bolsonaro was watching the returns. "They don't know what it was like under the dictatorship", he said. He has also promised to cut taxes and simplify the tax code, though he has not provided details.
In Sao Paulo, supporters celebrated on a main avenue with a giant inflatable doll in military uniform depicting Bolsonaro's running mate, retired general Hamilton Mourão. Haddad, the Workers' Party standard-bearer who was appointed by jailed ex-President Luiz Inacio da Silva, got 29 percent in the first round, and polls have predicted a close race in the runoff.