Jair Bolsonaro, Brazil's front-running far-right presidential candidate, underwent emergency surgery on Thursday after he was stabbed during a campaign event.

The 63-year-old lawmaker has "a wound in the abdomen that has been caused by a sharp instrument," said the Santa Casa hospital in the southeastern city of Juiz de Fora, describing his condition as "stable."

In a press conference after the surgery, doctors said the lawmaker had "three serious bowel perforations" that caused internal bleeding, adding that they had performed a colostomy and he was now stable.

Earlier, one of the candidate's sons, Flavio Bolsonaro, had announced on Twitter that his father's wounds were "superficial", but he later wrote: "Unfortunately, it's more serious than we thought."

"The perforation reached part of his liver, lung and intestine," Flavio Bolsonaro wrote. "He lost a lot of blood, arrived at the hospital ... almost dead. He appears to have stabilised now. Please pray!"

Jair Bolsonaro gestures after being stabbed in the stomach during a campaign rally in Juiz de Fora credit: RAYSA LEITE/AFP

Later, he said outside the hospital where his father was treated that he was conscious and the attack was a political boost.

"I just want to send a message to the thugs who tried to ruin the life of a family man, a guy who is the hope for millions of Brazilians: You just elected him president. He will win in the first round," said Flavio Bolsonaro.

Images shared on social media and Brazilian television showed Mr Bolsonaro being carried on the shoulders of a throng of supporters, before a man lunges at his stomach.

A military police spokesman told AFP a 40-year-old man had been arrested in connection with the assault. The suspect was carrying a knife wrapped in fabric, the spokesman said. The suspect was a former activist of a far-left political party.

Social media footage showed the apparent attacker being grabbed by several Bolsonaro supporters before the video cuts out.

In this video still provided by Fernando Goncalves, National Social Liberal Party presidential candidate Jair Bolsonaro is carried away after being stabbed credit: AP

Sometimes described as Brazil's Donald Trump, Mr Bolsonaro has a huge social media following of 8.5 million people.

The attack pushes an already chaotic campaign into further disarray.

Dr. Luiz Henrique Borsato, who operated on the candidate, said it could take two months for him to fully recover and he will spend at least a week in the hospital.

"His internal wounds were grave and put the patient's life at risk," Dr Borsato said, adding that a serious challenge now would be preventing an infection that could be caused by the perforation of Bolsonaro's intestines.

Bolsonaro, who has spent nearly three decades in Congress, is a law-and-order candidate who routinely says that Brazilian police should kill suspected drug traffickers and other criminals at will. His trademark pose at rallies is a "guns up" gesture with both hands to make them resemble pistols.

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He has openly praised Brazil's military dictatorship and in the past said it should have killed more people.

Mr Bolsonaro faces trial before the Supreme Court for speech that prosecutors said incited hate and rape. He has called the charges politically motivated.

President Michel Temer quickly condemned the attack and instructed his Minister of Security Raul Jungmann to reinforce security for candidates and conduct "a rigorous investigation," a spokesman for the presidency told AFP.

"It is intolerable to see that in a democratic state it is not possible to have a normal campaign," Mr Temer said.

Jair Bolsonaro is taken on the shoulders of a supporter moments before being stabbed credit: Antonio Scorza

Fernando Haddad, who will likely be the leftist Workers Party presidential candidate, said the stabbing was a "shame" and a "horror." Presidential rival Ciro Gomes also expressed outrage at the political violence.

A few people gathered outside the hospital where Mr Bolsonaro was treated Thursday night where the scene was calm.

Bruno Engler, 21, who is running for a Minas Gerais state congressional seat for Mr Bolsonaro's Social Liberal Party, was pacing outside the hospital with a few other Bolsonaro supporters. He said if he could, he would lynch the suspect police have in custody.

"They call us on the right the intolerant, the violent ones, but those who are intolerant and violent are them," Engler said, referring to leftist voters.

Brazilian President Michel Temer credit: EVARISTO SA/AFP

Next month's election is the most unpredictable since Brazil's return to democracy three decades ago. Political corruption investigations have jailed scores of powerful businessmen, politicians and alienated voters who are infuriated with their representatives.

With jailed former president Luiz Inacio Lula da Silva ruled out of the October 7 election, the latest polls from the Ibope Institute put Mr Bolsonaro in a clear lead with 22 percent compared with 12 percent each for environmentalist Marina Silva and center-left runner Ciro Gomes.

Under Brazilian campaign laws, Mr Bolsonaro's tiny coalition has almost no campaign time on government-regulated candidate ad blocs on TV and radio. That means he relies deeply on social media and raucous rallies around the country to drum up support. If Mr Bolsonaro is not able to go out in the streets, it could jeopardise his campaign.