On Jaira, Bolsonaro attacked the crowd in the southeastern state of Minas Gerais, reports the BBC. A suspect was arrested.
An elite team of doctors from Sao Paulo is en route to the city to take over the care for Bolsonaro, who leads first-round polling in the latest election surveys ahead of next month's elections.
Left-wing Lula had been the long-standing favourite but he is now in prison, appealing against a ban on his candidacy that was imposed after his conviction for corruption.
National Social Liberal Party presidential candidate Jair Bolsonaro flashes thumbs up at passersby in Brasilia's Ceilandia neighborhood, Brazil, Wednesday, Sept. 5, 2018. It is a surreal situation.
One of his campaign pledges has been to legalise the carrying of weapons in order to combat rising violent crime.
His vice presidential running mate, Hamilton Mourao, is a retired general who blamed leftists for the knife attack.
He described a painful wound, saying he had anxious about an attack on the campaign trail.
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How did the attack unfold?
Video footage showed Bolsonaro atop supporters' shoulders, clutching his stomach and grimacing after the assault. The candidate appears to grimace, then double over in pain.
"Unfortunately, (the injury) was more serious than we expected... was partially damaged liver, lung and intestines".
He arrived at the hospital "almost dead", Flavio wrote.
Bolsonaro underwent surgery and was later confirmed to be in stable condition.
His son, Flaviu Bolsonaro said on Twitter that after the incident, the father was hospitalized near death.
Updates with hospital statement in second paragraph.
Police say they have arrested a 40-year-old man identified as Adelio Obispo de Oliveria in connection with the stabbing.
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"His internal wounds were grave and put the patient's life at risk", said Dr. Luiz Henrique Borsato, who operated on the candidate, according to Reuters. He said the suspect was beaten badly by Bolsonaro supporters after the attack.
The president's predecessor, Dilma Rousseff, warned that the attack must not go unpunished. Naturally, this makes the USA media hate him.
Sometimes described as Brazil's Donald Trump, Mr Bolsonaro has a huge social media following of 8.5 million people. The suspect was a former activist of a far-left political party. He told Playboy magazine in 2011 that he would be "incapable of loving a gay son" and would rather see his son "die in an accident" than come out as gay.
In 2015, he was fined for saying in a newspaper interview that Congresswoman Maria do Rosario was "not worth raping; she is very ugly". He is being investigated for alleged comments he made against Afro-Brazilians, the outlet noted.
Who will be Brazil's next president?.
Ciro Gomes, labor candidate, showed solidarity for his political opponent and repudiated "violence as political language". Bolsonaro's rival candidates called off their campaign activities for Friday.
Mr Haddad was charged with corruption earlier this week and, like Lula, denies any wrongdoing. Lula appealed the electoral court decision that denied his candidacy, but on Thursday, Brazil's Supreme Court rejected it, effectively putting an end to the former president's hopes.
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