An opinion poll on Monday showed far-right presidential candidate Mr Bolsonaro has a 10-point lead over leftist Workers Party candidate Fernando Haddad.
Mr Bolsonaro increased his lead with 31 percent of voter support in the simulated first-round ballot.
The poll, conducted by Ibope, showed Mr Haddad had 21 percent.
If no candidate takes an overall majority – which seems likely – a runoff vote will be held on October 28.
Mr Bolsonaro and Mr Haddad were neck and neck on 42 percent each in the runoff opinion poll.
Ibope said support for centre-left candidate Ciro Gomes remained stable at 11 percent.
Business-friendly former Sao Paulo governor Geraldo Alckmin dropped to 8 percent.
The poll surveyed 3,010 voters across Brazil on September 29 and 30, and has a margin of error of two percentage points.
Who is Jair Bolsonaro?
Mr Bolsonaro, 63, is a former military officer.
He has been a member of the Chamber of Deputies since 1991 and is currently a member of the Social Liberal Party (PSL).
Mr Bolsonaro is deeply controversial in Brazilian society.
He was stabbed at a rally in September, and was allegedly seriously injured, though he is said to be recovering well.
His son tweeted that the stab wound reached parts of his liver, lung and intestines.
Mr Bolsonaro has now been released from hospital, but is not expected to be able to return to campaigning before the first round of votes.
Both his supporters and critics condemned the violent attack.
He has sparked outrage with homophobic and misogynist comments.
Mr Bolsonaro once told a congresswoman she was not worth raping and has equated homosexuality with paedophilia.
He has also advocated the death penalty.
In a public speech in April 2017, he said he had five children, the first four were male and that for the fifth he produced a daughter out of “a moment of weakness”.
Mr Bolsorano has a massive support base in the country’s millions of evangelical Christians who praise his anti-abortion stance.
Protests against the candidate have taken place all across Brazil.