Flavio Bolsonaro, who initially described his father's injuries as superficial, then tweeted that the 63-year-old man suffered injuries to his liver, lungs and intestine and that he had "lost a lot of blood."
"Sadly it is more serious than we thought," he said, while his father underwent emergency surgery at a nearby hospital in Juiz de Fora, a town of 700,000 people in the state of Minas Gerais.
The hospital itself, quoted by the G1 website, said that Bolsonaro suffered a perforated bowel and described his condition as "stable".
The images shared on social networks and on Brazilian television showed that Bolsonaro was carried on the shoulders of a crowd of followers before a man threw himself on his stomach.
A spokesman for the military police told AFP that a 40-year-old man had been arrested in connection with the assault. The suspect had a knife wrapped in cloth, the spokesman said.
The footage from the social networks showed that several Bolsonaro attackers grabbed the apparent attacker before the video was cut.
Other images showed that the presidential candidate, a former army captain, was taken to a safe place and Bolsonaro's son initially said his father was "fine."
With former President Luiz Inacio Lula da Silva excluded from the October 7 elections, the latest Ibope Institute polls put Bolsonaro on a clear lead with 22 percent compared to 12 percent for environmentalist Marina Silva and the runner from center-left Ciro Gomes.
One of his campaign promises has been to legalize the carrying of arms to combat the growing violent crime.
Despite being a member of Congress for a long time, he has successfully presented himself as a stranger, untouched by the corruption scandals that engulf a large part of the political elite.
Perhaps the message that goes the furthest is Bolsonaro's impulse for a stronger repression against crime, in a country where the police already often participate in low-level wars against gangs. Around 64,000 people die in homicides each year.
However, he has also courted the deep controversy with the comments that attack women and sexual minorities, as well as praise the country's 1964-1985 military dictatorship.
Sometimes described as the Brazilian Donald Trump, Bolsonaro has a large following in the social networks of 8.5 million people.
The only Brazilian politician currently most popular is the former leftist president Luiz Inacio Lula da Silva, who is imprisoned for corruption and has been banned from competing in the October elections.