Videos posted online showed Bolsonaro on Thursday was being carried on his supporters' shoulders in a large crowd, when someone quickly stabbed him in the abdomen.
Bolsonaro was then carried to safety but appeared to be unconscious, Xinhua reported.
Police promptly arrested the suspect identified as 40-year-old Adelio Bispo de Oliveira, who was immediately grabbed by Bolsonaro's supporters.
There were conflicting reports as to the seriousness of the injury.
National news network Globo reported Bolsonaro required emergency surgery, while his son, Flavio, a candidate for the Brazilian senate, originally said on Twitter that his father's wounds were "only superficial."
Flavio has since called on supporters to pray for his father.
"Unfortunately it was more serious than we had expected. The perforation reached the liver, lungs and intestines. He lost a lot of blood, arrived at the hospital ... almost died. His condition now seems stabilized," he said.
Brazilian President Michel Temer described the attack as "intolerable."
The other presidential hopefuls expressed their shock at the attack on Bolsonaro, who leads with some 20-percent support among 12 candidates, according to polls.
Up to last week, ex-president Luiz Inacio Lula da Silva was leading with some 30-per cent support, but election authorities disqualified him from running due to his conviction of corruption charges.
Ciro Gomes of the Democratic Labor Party called the attack "barbaric," and said that he rejects "violence as a political language and is in solidarity with his opponent."
Fernando Haddad of the Workers' Party, who was Lula's running mate and a substitute candidate, said he opposes "any act of violence."
Meanwhile, Geraldo Alckmin of the Brazilian Social Democracy Party said "politics is made with dialogue and conversation, not hate ... any acts of violence are deplorable."
The attack against Bolsonaro "represents a double attack against his physical integrity and against democracy," said Marina Silva of the Brazilian Sustainability Network.
(This story has not been edited by Social News XYZ staff and is auto-generated from a syndicated feed.)
