With 92 percent of votes counted, Bolsonaro received 47 percent of valid ballots, far ahead of Haddad's 28 percent but short of the outright majority needed to avoid an October 28 runoff.
In what is likely to be a deeply polarizing runoff, Bolsonaro, an outspoken apologist for Brazil's 1964-1985 military dictatorship, will now face leftist Fernando Haddad, the former mayor of Sao Paulo, in a second round of voting on October 28.
Monica de Bolle, the director of Latin American studies at Johns Hopkins University, said she expected Bolsonaro to come under heavy fire from Haddad and other defeated presidential candidates, above all for his disdain for democracy.
Many voters are deeply distrustful of the PT and the party's figurehead, Lula, as a result of their involvement in corruption scandals in recent years.
"Anyone who lives in Brazil knows that life got worse under the Workers' Party, especially for the middle class", she said.
Jair Bolsonaro, whose last-minute surge nearly gave him an electoral stunner, had 46pc compared to 29pc for former Sao Paulo Mayor Fernando Haddad, according to figures from Brazil's Superior Electoral Tribunal with 99.9pc of the vote counted.
Some Bolsonaro supporters called on him to moderate his message to ensure victory, but the candidate said he would not ease up on an anti-establishment message that has resonated with voters. Much of Haddad's support is concentrated in northeast Brazil, which is often slower to report electoral results.
Google shuts down Google
Names, email addresses, occupation, gender, and age were all accessible, given that developers knew how to make use of the flaw. Secondly, access to the Gmail API is going to be limited to only those apps that directly enhance email functionality.
Senna said he was anxious that Bolsonaro's presidential rivals would gang up on him and back Haddad in the runoff.
Asked whom he would endorse, Gomes referred to the slogan of anti-Bolsonaro protesters in recent weeks: "Not him, certainly".
At a news conference afterward, Haddad cast the second round as pitting Bolsonaro's "neoliberalism" against the social programs that the PT has promoted.
Brian Winter, the editor-in-chief of Americas Quarterly magazine, said the results underscored "the total disappearance of the Brazilian centre" and that Bolsonaro seemed nearly certain to glide to victory.
Haddad can only win in the second round if he converts sceptics, galvanises Sunday's vanquished centrist candidates and their supporters and goes after his opponent on policy issues such as crime and security which, until now, he has appeared unwilling to grapple with, analysts said.
"Bolsonaro has an advantage over Haddad, he's in a growth curve. for him to lose from here, it's not about Haddad getting it right, it's about him making mistakes", he added.
"A lot of young people are voting for him".
Prior to Sunday's first-round vote, about 41 percent of Brazil's electorate said they wouldn't vote for Haddad under any circumstances, according to Datafolha polling institute. He has also promised to cut taxes and simplify the tax code, though he has not provided details. "If the PT is voted into power and there is a military intervention, I would support it".